Technical Field [0001] The present invention relates to an air motor mounted on a spindle device that is used in an electric painting process, or on a drive member of a spindle system for a machine tool, which uses small tools in diameter needed for high velocity revolution, for example, and an electric painting device. Background Art [0002] The air motor is an engine for rotating a main shaft by having the main shaft supported by static pressure gas bearings, and ejecting a gas such as compressed air toward an impeller (rotor blade) from a nozzle (holes and tubes), and is widely used, mounted on electric painting devices, high-precision machine tools, and similar devices. Various modifications of conventional devices have been made so as to improve rotation efficiency, and various motor configurations as concrete examples thereof are well-known (See Patent Document 1 and Patent Document 2). [0003] FIGS. 1 and 2 illustrate a configuration of an air motor (spindle device with air turbine) mounted on an electrostatic spray gun of an electric painting device as a configuration example of such an air motor. This air motor includes a hollow main shaft 2, which extends in an approximately right circular tube form from a base to a tip (from right end to left end in FIG. 1) , and an impeller 4, which is arranged on the base of the main shaft 2 concentric therewith. The impeller 4 includes a annular portion 6, which is a larger flat plate in diameter than the main shaft 2 and is positioned and fixed to the base of the main shaft 2 by a fastening member or the like, and an impeller main body 8, which is a short cylinder that is larger in diameter than the main shaft and smaller in diameter than the annular portion 6 and is fixed on an axial side (right side in FIG. 1) of the annular portion 6. Multiple turbine blades 10 are formed across the entire impeller main body 8 at equal intervals along the circumference thereof. Each of the turbine blades 10 is structured with the same form so as to have the same gradient (for example, forward tilting in normal rotative direction (right rotative direction C in FIG. 2) of the impeller 4) in the same rotative direction. [0004] The main shaft 2 and the impeller 4 making such a structure are rotatably supported by predetermined bearings (radial static pressure gas bearings 14 and axial static pressure gas bearings 16) in a housing 12, respectively. In the structure shown in FIG. 1, a bearing main unit 18 of the radial static pressure gas bearings 14 is made of a porous material in cylindrical form, fixed at a central portion along the axis inside of the housing 12, and arranged such that the inner periphery thereof is arranged facing a central portion along the axis of the external surface of the main shaft 2 at a slight gap therefrom. An air supply channel 20, which supplies compressed air in spaces between the housing 12 and the periphery of the main shaft 2 via the bearing main unit 18, is provided inside of the housing 12 and extends to the external surfaces of the bearing main unit 18 of the radial static pressure gas bearings 14 . Meanwhile, the axial static pressure gas bearings 16 are structured such that a bearing main unit 22 thereof made of a porous material is ring-shaped and has an oblong cross section, fixed to the base (right end in FIG. 1) of the housing 12, and is arranged such that an axial side (right side in FIG. 1) faces the circumference of the opposite side (left side) to the fixing side of the annular portion 6, which comprises the impeller 4, for the impeller main body 8. The air supply channel 20 extends to the external surface of the bearing main unit 22 of the axial static pressure gas bearings 16 so as to also supply compressed air to spaces from a side of the annular portion 6 of the impeller 4 via the periphery of the bearing main unit 22. [0005] When rotatably supporting the main shaft 2 and the impeller 4 using the radial static pressure gas bearings 14 and the axial static pressure gas bearings 16, compressed air is continuously provided in the gaps between the bearing main bodies 18 and 22, the main shaft 2, and the impeller 4 (the annular portion 6) via the air supply channel 20, the radial static pressure gas bearings 14, the axial static pressure gas bearings 16, and the bearing main bodies 18 and 22. The compressed air supplied to the spaces is blown continuously on a side of the annular portion 6 and the external surface of the main shaft 2, forming a film of air in all of the spaces due to the compressed air. As a result, the 3 main shaft 2 and the impeller 4 keep a noncontact state with the bearings 14 and 16 via the film, and are supported rotatably by the bearings 14 and 16. [0006] Note that the compressed air continuously supplied to 3 the spaces through the air supply channel 20 is successively exhausted to the exterior space via exhaust holes 24, which are provided within the bearing main body 18 of the radial static pressure gas bearings 14, an exhaust channel 26, which is provided within the housing 12, and spaces within the housing 5 12. In the case of mounting an air motor {spindle device with air turbine), whichmakes such a structure, on an electrostatic spray gun of an electric painting device, the impeller 4 and the main shaft 2 to which the impeller 4 is fixed should be aligned along the axis by other axial static pressure gas 0 bearings {not illustrated in the drawing), which are additional ones to the axial static pressure gas bearings 16, rotatably supporting the opposite side (i.e., the fixing side for the impeller main body 8 {right side in FIG. 1)) to the supporting side of the annular portion 6, which is 5 supported by the axial static pressure gas bearings 16. [0007] Moreover, the impeller 4 is arranged in the housing 12 such that the inner periphery on the base side (right end side in FIG. 1) and the outer periphery of the impeller main body 8 may face each other all around. In other words, the base side inner periphery of the housing 12 is positioned radially outward from the impeller main unit 8. [0008] Multiple {for example, six holes at equal intervals in the structure illustrated in FIG. 2) turbine air nozzle holes 28, which are formed at predetermined intervals along the circumference toward the periphery of the impeller main body 8, are formed on the base side of the housing 12, which is positioned radially outward from the impeller main body 8. These turbine air nozzle holes 28 are formed such that all centers thereof are positioned within a virtual plane orthogonal to a central axis of the housing 12, and tilt at the same angle with respect to the radial direction of the housing 12 (in other words, they forward-tilt in the normal rotative direction {right rotative direction C in FIG. 2) of the impeller 4.) Furthermore, these turbine air nozzle holes 28 continue to a turbine air supply channel 30, which has an opening 28u on an upstream end (compressed air {turbine air) supply source side) formed all around near the base side periphery of the housing 12, and the turbine air supply channel 30 continues to a turbine air supply opening 32, which opens to the base {right end in FIG. 1) of the housing 12 at one place along the circumference. Meanwhile, the respective turbine air nozzle holes 28 have downstream ends (turbine air spray inlets) 28d open to the base side inner periphery of the housing 12. In other words, the downstream ends (turbine air spray inlets) 28d of the respective turbine air nozzle holes 28 are formed closely facing the multiple turbine blades 10 formed on the external surface of the impeller main unit 8. [0009] Furthermore, a brake air nozzle hole 34 is formed in the housing 12, opening to the periphery of the impeller main body 8 such that it does not overlap with the above multiple turbine air nozzle holes 28 on the base side. The brake air nozzle hole 34 is formed such that the center thereof is positioned within a virtual plane having the same central axis as the turbine air nozzle holes 28 (i.e., within a virtual plane orthogonal to the central axis of the housing 12 that is the same as those of the turbine air nozzle holes 28) and tilts at a predetermined angle (approximately the same angle as the turbine air nozzle holes 28) in the opposite direction than the turbine air nozzle holes 28 with respect to the radial direction of the housing 12 (in other words, forward-tilts in reverse rotative direction of the impeller 4 (left rotative direction A in FIG. 2) ) . Moreover, the brake air nozzle hole 34 has an upstream end (brake air supply source side) opening 34u continuing to a brake air supply opening 36, which opens to the base (right end in FIG. 1) of the housing 12, and a downstream end (brake air spray inlet) 34d opening on the base side inner periphery of the housing 12. In other words, the downstream end (brake air spray inlet) 34d of the brake air nozzle hole 34 is formed closely facing themultiple turbine blades 10 formed on the external surface of the impeller main body 8. [0010] Note that a circular rotation detecting sensor 38 is arranged on the base side of the housing 12 such that the 5 inner periphery of the bearing main unit 22 of the axial static pressure gas bearings 16 and the other axial side (left side in FIG. 1) of the impeller main body 8 may face each other with a predetermined distance therebetween. The rotation detecting sensor 38 includes a detector (right side portion 10 in FIG. 1) capable of facing the other axial side of the impeller main body 8 and a to-be-detected unit (encoder) on the other side of the impeller main unit 8. This constitutes a sensor mechanism for detecting rotational state (rotation speed, rotative direction, and the like) of the impeller 4. With 15 the sensor mechanism, the rotational state (rotation speed, rotative direction, and the like) of the impeller 4 is detected by detecting and measuring positional change of the to-be-detected unit (encoder) using the detector. [0011] 20 A magnet, for example is employed as the rotation detecting sensor 38 in the air motor illustrated in FIG. 1. This is because the axial bearing 16 is provided only on the output side of the rotary movement, as shown in FIG. 1, which may allow the main shaft 2, the impeller 4, and the impeller 25 main body 8 to slip out to the opposite side to the output side (opposite direction than the output side of the rotary movement) of the rotary movement. Employment of the magnet for the rotation detecting sensor 38 thereby allows attraction to the main shaft 2 so as to reduce the chance of the main shaft 2, the impeller 4, and the impeller main body 8 from slipping out to the opposite side to the output side of the rotary movement. In this manner, as long as the rotation ) detecting sensor 38 can suppress the possibility mentioned above, function and arranging position may be appropriately selected according to purpose. For example, installation of the axial bearings 16 on either side of the impeller 4 allows a structure not employing a magnet as the rotation detecting ) sensor 38. [0012] When coating using an electrostatic spray gun of an electric painting device on which the air motor (spindle device with air turbine) making such a structure is mounted, the 5 air motor operates in the following manner. As described above, the main shaft 2 and the impeller 4 are rotatably supported on the housing 12 by the radial static gas bearings 14 and the axial static gas bearings 16, respectively. In this state, compressed air (turbine air) ) is supplied to the multiple turbine air nozzle holes 28 via the turbine air supply opening 32 and the turbine air supply channel30. Thesuppliedcompressedair (turbineair) isblown onto the multiple turbine blades 10 formed on the periphery of the impeller main unit 8 from the downstream ends (turbine 5 air spray inlets) 28d of the respective turbine air nozzle holes 28. As a result, the turbine blades 10 are continuously depressed in their tilt direction, namely normal rotative direction (right rotative directionC in FIG. 2) of the impeller 4, rotating the impeller 4 and the main shaft 2 in the normal rotative direction at a predetermined rotation speed (e.g., several tens of thousands rpm). [0013] Acoatingmaterial is then supplied into a predetermined cup (not illustrated in the drawing) via a coating material supply-pipe (not illustrated in the drawing) inserted inside of the main axis 2 in this state. The cup is fixed to a portion of the front end (left end in FIG. 1) of the main shaft 2 that protrudes (is exposed) to the outside of the housing 12, and is negatively charged. As a result, the coating material supplied to the cup is made into ion microparticles within the cup that rotates at a high speed along with the main shaft 2. [0014] The coating material made into ion microparticles is thrown toward a positively-charged surface to be coated utilizing electrostatic attraction and adhered on that surface. Note that the compressed air (turbine air) blown onto the respective turbine blades 10 is exhausted out into the outside space from an opening on the base side of a circular space 40 between the inner periphery on the base side of the housing 12 and the outer periphery of the impeller main body 8 via an exhaust channel (not illustrated in the drawing) connecting to the opening. [0015] On the other hand, in the case of stopping the coating operation on the surface to be coated, supply of compressed air (turbine air) to the respective turbine air nozzle holes 28 and supply of the coating material to the cup are stopped, and compressed air (brake air) is supplied to the brake air nozzle hole 34 via the brake air supply opening 36. The 5 supplied compressed air (brake air) is blown onto the multiple turbine blades 10 from the downstream end (turbine air spray inlet) 34d of the brake air nozzle hole 34. As a result, the turbine blades 10 are continuously depressed in the opposite direction of their tilt direction, namely reverse direction 10 (left rotative directionAin FIG. 2) of the impeller 4, thereby imposing a negative load on the inertia rotation in the normal rotative direction of the impeller 4 and the main shaft 2 so as to halt early. [0016] 15 Then, once the rotation detecting sensor 38 has detected that the rotation speed of the impeller 4 and the main shaft 2 has slowed down and rotation thereof completely stops, supply of compressed air (brake air) to the brake air nozzle hole 34 is then stopped. 20 Note that even in this case, the compressed air (brake air) blown onto the respective turbine blades 10 is exhausted out to the outside space from the opening on the base side of the circular space 40. [0017] 25 However, driving force of the air motor is dependant on momentum of the jet flow from the nozzle that hits a turbine, namely momentum of the compressed air (turbine air) ejected from the downstream ends (turbine air spray inlets) 28d of the turbine air nozzle holes 28 to be blown onto the multiple turbine blades 10 that are formed on the periphery of the ' i impeller 4 (more specifically, the impeller main body 8). The driving force (torque) of the impeller 4 sprayed with the compressed air (turbine air) at that time is calculated using the following Equation 1 (See Non-patent Document 1) . Note that in Equation 1, T denotes torque of the turbine (the impeller 4), F denotes momentum (driving force) of jet flow (ejected compressed air from the turbine air nozzle holes 28) from the nozzle, R denotes radius of the turbine (the impeller 4 sprayed with the ejected compressed air) on which the jet flow impacts, m denotes mass (where mass flow rate x At) of the jet flow (ejected compressed air), V denotes flow velocity of the jet flow (the ejected compressed air) , and Vt denotes circumferential velocity (where Vt is 2nRN and N denotes motor rotation frequency) at the region (region of the impeller 4 on which the jet flow impacts) impacted by the jet flow. [0018] [Equation 1] T = F-R = m(V-Vt)R - (1) [0019] The flow velocity of the gas flowing into the nozzle (flow velocity of the compressed air (turbine air) immediately after being supplied to the turbine air nozzle holes 28 from the turbine air supply channel 30 via the upstream end openings 28u or inlet to the turbine air nozzle holes 28; hereafter it is referred to as inlet flow velocity) is not acoustic velocity even under choked conditions such that maximum velocity as jet flow is attained in the nozzle, and is calculated using the following Equation 2. Note that in Equation 2, ve denotes inlet flow velocity in the nozzle (the turbine air nozzleholes 28) in a choked state, aodenotes acoustic velocity, and k denotes specific heat ratio of compressed air (turbine air) . [0020] [Equation 2] v* = a0 /TT7 (about 313m/s) ••• (2) e V k+1 [0021] Moreover, mass (namely, maximum value of mass flow rate) of the jet flow (ejected compressed air) in the above choked state is calculated using the following Equation 3 . Note that in Equation 3, mmax denotes mass of the jet flow (ejected compressed air) in the above choked state, po denotes density of the compressed air (turbine air) on the upstream side, andAe denotes inlet area of the nozzle (the turbine air nozzle holes 28). [0022] [Equation 3] k+1 mmax" \k+W p°aoA° "' (3) [0023] where if specific heat ratio (k) is 1.40, isopiestic specific heat Cp is 1007 (J/kg»K), and temperature of the compressed air (turbine air) on the upstream side is T (K) , the acoustic velocity (ao) is represented by the following Equation 4. [0024] [Equation 4] a0= VcpT *"(4) [0025] Furthermore, the density (p0) of the compressed air (turbine air) on the upstream side is calculated using the following Equation 5. Note that in Equation 5, P0 denotes pressure of the compressed air (turbine air) on the upstream side. [0026] [Equation 5] '••"•» ^-unSnr ■■•« [0027] In light of the above, in order to improve driving efficiency of the air motor, the inlet flow velocity (ve) {approximately 313 m/s) of the compressed air (turbine air) in the nozzle {the turbine air nozzle holes 28) in a choked state should be raised to the acoustic velocity {340 m/s) . For example, expanding the compressed air {turbine air) using pressure drop in the compressed air by fluid friction {inner periphery of the turbine air nozzle holes 28) of the nozzle makes it possible to increase the inlet flow velocity (ve) . However, even in this case, the maximum velocity is acoustic velocity {340 m/s). [0028] Making the inlet flow velocity (ve) be the acoustic velocity through flow velocity increase is achieved in the case where length of the nozzle is set to L or greater, which is represented in Equation 6 {see Non-patent Document 2) given below when Mi is ve/ a0. Note that in Equation 6, rh denotes hydraulic radius {inner radius in the case of round holes or circular tubes, cross-sectional area A in the case of square holes and square tubes, and is defined by 2 x A /C in the case where circumference length is C) , and Cf denotes viscous friction factor of the wall {inner periphery of the turbine air nozzle holes 28) of the nozzle {holes and tubes) . At that time, the viscous friction factor {Cf) is given as 0.0576 x Re"0"2 using the Reynolds number {Re = vD/v) when v denotes flow velocity of compressed air, D denotes diameter {inner diameter) of the nozzle {holes and tubes), and v denotes kinematic viscosity. In this manner, Equation 6 holds true even when the cross-sectional shape of the nozzle {the turbine air nozzle holes 28) is other shapes than round, such as square. [0029] [Equation 6] L" 2c, I kM,2 2k ,n\ 2+(k-1)Mf // W Prior Art Documents Patent Documents [0030] Patent Document 1: JP 2006-300024 A Patent Document 2: JP 2009-243461 A Non-patent Documents [0031] Non-patent Document 1: Yukio Tomita, 'Hydraulics', Jikkyo ShuppanCo., Ltd., 1982, p. 224 Non-patent Document 2 : YasuoMori, Syoji Isshiki, HaruoKawada, 'Introduction to Thermodynamics', Yokendo, Co., Ltd., 1989, p. 214 Summary of Invention i Problem To Be Solved [0032] As described above, in order to improve driving efficiency of the air motor, the inlet flow velocity (ve) of the compressed air in the nozzle (holes and tubes) in a choked state should be raised to be close to the acoustic velocity (34 0 m/s). In other words, in designing the nozzle (the turbine air nozzle holes 28) of the air motor, it is considered effective to set the length of the nozzle to at least the value (namely L) calculated by Equation 6 in accordance with the inlet flow velocity (v_) in the nozzle calculated from the maximum torque required by the air motor, diameter size (hydraulic radius) (rh) of the nozzle, and supply source conditions for the compressed air (specifically, supply pressure (po) or supply flow rate). [0033] However, no technology for optimum design of the nozzle based on the inlet flow velocity (ve) in the nozzle, diameter size (hydraulic radius) (rh) of the nozzle, and supply conditions for the compressed air (supply pressure (p0) or supply flow rate) so as to improve driving efficiency of the air motor is not yet currently known. [0034] j The present invention has been devised so as to resolve j such problems, and an object thereof is to provide an air motor improving drive efficiency by setting length of a nozzle based on compressed air inlet flow velocity in the nozzle (holes and tubes) , which supplies compressed air to be blown onto turbine blades of an impeller, diameter size (hydraulic radius) of the nozzle, and supply conditions for the compressed air (supply pressure or supply flow rate). Solution to the Problem [0035] In order to achieve such an object, an air motor according to an embodiment of the present invention includes a housing, a main shaft inserted inside of the housing, an impeller fixed concentrically with the main shaft to an inserted portion of the main shaft inside of the housing and having multiple turbine blades formed on the outer periphery, bearings for rotatably supporting the main shaft and the impeller in the housing, and at least one nozzle having a tubular or hole-shaped channel for ejecting compressed air to the respective turbine blades for rotating the impeller along the circumference. With this air motor, when Mi = v./ a,, where r, denotes hydraulic radius of the channel oft he nozzle, cr denotes viscous friction factor of a wall of the channel, k denotes specific heat ratio of compressed air, ve denotes flow velocity of the compressed air in an entrance of the channel, and a., denotes acoustic velocity, L is calculated using [0036] [Equation 7] rh /1-Mi2 , k+1 , I (k+PM? \\ f . L= ^li^+irnTKi^w)) '"(6) and the channel of the nozzle has a length set to a dimension of the calculated value of L or greater. [0037] Note that while the channel of the nozzle should be set to the dimension of the calculated value of L or greater, i it is preferable to set it to a predetermined dimension of five times the calculated value of L or greater at that time. Moreover, the bearings are preferably static pressure gas bearings. Furthermore, of the bearings, at least bearings on one end side are preferably structured as ceramic roller bearings . Yet further, the roller bearings preferably include a r aceway ring on one side mounted on the ho us mg, and a raceway ring on the other side mounted on a spindle facing the raceway ring on the one side, and a plurality of rolling elements incorporated between these raceway ring, where [0038] either the bearing rings or the rolling elements or all of them are made of ceramics. Yet even further, it is preferable that either the bearing rings or the rolling elements or all of them are made of non-conducting ceramics. Yet even further, it is preferable that the bearing rings and the rolling elements are made of conducting ceramics . Yet even further, an electric painting device of the present invention includes the air motor of any of the above configurations. Advantageous Effect of the Invention [0039] According to the present invention, an air motor improving drive efficiency by setting length (nozzle length) of a nozzle based on compressed air inlet flow velocity in the nozzle, which supplies compressed air to be blown onto turbine blades of an impeller, diameter size (hydraulic radius) of the nozzle, and supply conditions for the compressed air (supply pressure or supply flow rate), and an electric painting device may be implemented. Brief Description of the Drawings [ Ct 4 0 | FIG. 1 is a cross-sectional view illustrative of a structure of an air motor according to an embodiment of the present invention; FIG. 2 is a cross-sectional view of the air motor shown in FTG. 1 cut along line Fl-Fl; FIG. 3 is a diagram showing supply pressure of compressed air to nozzle length in the case where compressed air is made to flow through a nozzle with a diameter (inner diameter) of 1.1 mm at a rate of 20 NL/ min, and supply pressure ratio to a reference supply pressure; FIG. 4 is a diagram showing supply pressure of compressed air to nozzle length in the case where compressed air is made to flow through a nozzle with a diameter (inner diameter) of 1.1 mm at a rate of 50 NL/ min, and supply pressure ratio to a reference supply pressure; FIG. 5 is a diagram showing supply pressure of compressed air to nozzle length in the case where compressed air is made to flow through a nozzle with a diameter (inner diameter) of 1.8 mm at a rate of 50 NL/ min, and supply pressure ratio to a reference supply pressure; FIG. 6 is a diagram showing supply pressure of compressed air to nozzle length in the case where compressed air is made to flow through a nozzle with a diameter (inner diameter) of 1.8 mm at a rate of 150 NL/ min, and supply pressure ratio to a reference supply pressure; FIG. 7 is a diagram showing supplypressure of compressed air to nozzle length in the case where compressed air is made to flow through a nozzle with a diameter (inner diameter) of 2.5 mm at a rate of 150 NL/ min, and supply pr-i_-sure ratio to a reference supply pressure; FIG. 8 is a diagram showing supply pressure c i compressed air to nozzle length in the case where compressed air is made to flow through a nozzle with a diameter (inner diameter) of 2.5 mm at a rate of 300 NL/ min, and supply pressure ratio to a reference supply pressure; FIG. 9 is a cross-sectional view schematically illustrating an entire structure of a spindle device using an air motor according to another embodiment; FIG. 10 is a cross-sectional view schematically illustrating an entire structure of a spindle device using an air motor according to another embodiment; and FIG. 11 is a cross-sectional view illustrating an enlarged structure around a ceramic ball bearing of a spindle device using an air motor according to another embodiment. Description of Embodiments [0041] Embodiments including an air motor of the present invention will now be described with reference to the attached drawings. Note that while the air motor according to this embodiment may be assumed to be mounted on a spindle device that is used in an electric painting process, or on a drive member of a main spindle system for a machine tool, that uses small tools in diameter needed for high velocity revolution, for example, the mounting instrument is not limited thereto. [0042] Moreover, ~!ie air motor according to this embodiment limits length : ihe nozzle that constitutes the air motor to a dimens :.'■:". within a predetermined range, and there is no problem for v he basic configuration of the air motor other 5 than the nozzl e r.w be that of a well-known air motor. Therefore, the configuration (FIGS. 1 and 2) of the air motor (spindle device with air turbine) mounted on an electrostatic spray gun of an electr it; painting device as described above is assumed as a motor configuration example, where this embodiment is 0 described on the premise of this motor configuration. [0043] The air motor according to this embodiment includes the housing 12, a main shaft 2, which is inserted inside of the housing 12, the impeller 4, which is fixed to a portion 5 of the main shaft 2 inserted inside of the housing 12 concentrically with the main axis 2 and has the multiple turbine blades 10 formed on the outer periphery, static pressure gas bearings (the radial static pressure gas bearings 14 and the axial static pressure gas bearings 16) for rotatably ;0 supporting the main axis 2 and the impeller 4 in the housing 12, and at least one of nozzles 28 and 34 having tubular or hole-shaped channels for ejecting compressed air to the respective turbine blades 10 for rotating the impeller 4 along the circumference. :5 [0044] As described above, while the air motor illustrated in FIGS. 1 and 2 is assumed as an example configuration in this embodiment, the housing 12, the main axis 2, the impeller 4, and the static pressure gas bearings (the radial static pressure gas bearings 14 and the axial static pressure gas bearings 16) are not particularly limited to the illustrated configuration of the drawings, and may be modified appropriately in accordance with intended purpose and use conditions of the air motor. For example, configuration of the housing 12 and the main shaft 2, size and number of the impeller 4, configuration and number of the turbine blades 10 formed on the impeller main body 8 of the impeller 4, arranging position and number of the static pressure gas bearings 14 and the axial static pressure gas bearings 16 need to be respectively set arbitrarily in accordance with intended purpose and use conditions of the air motor. [0045] In the configuration given in FIGS. 1 and 2, the turbine air nozzle holes 28 are formed such that all centers thereof are positioned within the same virtual plane {hereafter referred to as turbine air nozzle hole formation plane) that is orthogonal to the central axis of the housing 12, and tilt ( forward-tilt in the normal rotative direction (right rotative direction C in FIG. 2) of the impeller 4) at the same angle with respect to the radial direction of the housing 12. In this case, the turbine air nozzle holes 28 are formed on the base side of the housing 12 as holes opening to the outer periphery of the impeller 4 ( impeller main body 8) , and include hole-shaped channels for spraying compressed air (turbine air) to the respective turbine blades 10 so as for the impeller 4 to rotate along the circumference (normal rotative direction C) . [0046] Moreover, the brake air nozzle hole 34 is forced such that the center thereof is positioned within the sane plane 5 as the turbine air nozzle hole formation plane, and tilt (forward-tilt in reverse rotative direction (left rotative i i direction A in FIG. 2) of the impeller 4) at a predetermined angle (forexample, approximately the same angle as the turbine air nozzle holes 28) in the opposite direction than the turbine 0 air nozzle holes 28 with respect to the radial direction of the housing 12. In this case, the brake air nozzle hole 34 is formed on the base side of the housing 12 as holes opening j i to the outer periphery of the impeller 4 (impeller main body 8) so as not to overlap with the turbine air nozzle holes 5 28, and includes a hole-shaped channel for spraying compressed air (brake air) to the respective turbine blades 10 so as for the impeller 4 to rotate along the circumference (reverse rotative direction A). In other words, the turbine air nozzle holes 28 and 0 the brake air nozzle hole 34 are respectively configured as a nozzle of the air'motor. [0047] Note that arranging position, number, and cross-sectional form of the turbine air nozzle holes 28 and 5 the brake air nozzle hole 34 may be arbitrarily set. For example, while FIGS. 1 and 2 illustrate a configuration of an air motor in which six of the turbine air nozzle holes 28 are formed such that centers thereof are positioned and open in the same turbine air nozzle formation plane at equal intervals on the base side of the housing 12 toward the outer periphery of the impeller 4 (impeller main body 8), a configuration in which the same or a different number of turbine air nozzle holes 28 are formed such that the centers thereof | are positioned in multiple turbine air nozzle hole formation planes is possible. Moreover, while FIGS. 1 and 2 illustrate a configuration of an air motor in which only a single brake air nozzle hole 3 4 is formed, a configuration in which multiple brake air nozzle holes 34 are formed with the same aspects (except for tilt direction) as any of the above turbine air nozzle holes 28 is also possible. Furthermore, while FIGS. 1 and 2 illustrate a configuration of an air motor in which the turbine air nozzle holes 28 and the brake air nozzle hole 34 are formed as round holes with circular cross-sectional forms, a configuration in which the turbine air nozzle holes 28 and the brake air nozzle hole 34 are formed as square holes with square (polygon such as a quadrangle) cross-sectional forms is also possible. [0048] Yet even further, while FIGS. 1 and 2 illustrate a configuration of a nozzle (the turbine air nozzle holes 28 and the brake air nozzle hole 34) having hole-shaped channels for ejecting compressed air (turbine air or brake air) to the respective turbine blades 10 so as for the impeller 4 to rotate along the circumference (either normal rotative direction C or reverse rotative direction A) , the nozzle may have tubular (for example, a round or square (polygon such as a quadrangle) cross-sectional form) channels. [0049] Length of the channels of the nozzle (distance (distances Lt and Lb in FIG. 1) from the upstream end openings > 28u and 34u until the downstream end openings 28d and 34d) is set to a dimension of at least L, which is calculated using the following Equation 6. Note that in Equation 6, rh denotes hydraulic radius (2 x nr:/2nr = r (inner radius) when inner radius is r) of the nozzle (the turbine air nozzle holes 28 ) and the brake air nozzle hole 34), and c± denotes viscous friction factor of the wall (inner periphery of the turbine air nozzle holes 28 and the brake air nozzle hole 34) of the nozzle. At that time, the viscous friction factor (Cf) is given as 0.0576 x Re - 0.2 using the Reynolds number (Re = i vD/v) when v denotes flow velocity of compressed air (turbine air and brake air), D denotes diameter (inner diameter) of the nozzle (the turbine air nozzle holes 28 and the brake air nozzle hole 34), and v denotes kinematic viscosity. [0050] ) Equation 8 rh /1-M2 k+1 . / (k+1)M,2 \\ . . .. [0051] Nozzle lengths (nozzle length Lt of the turbine air 5 nozzle holes 28 and nozzle length Lb of the brake air nozzle hole 34) of the nozzle are not particularly limited and may be arbitrarily set in accordance with intended purpose and use conditions of the air motor as long as it is set to at least the calculated value L using Equation 6. As an example, this embodiment assumes a case where the nozzle lengths Lt and Lb of the nozzle (28 and 34) are set to predetermined > dimensionof 5 times or more (5L In the case where insulation between the housing 102 and the main shaft 104 is required, any or all of the outer rings 108a and 110a, the inner rings 108b and 110b, and the rolling members (balls) 116 and 118 should be made of non-conducting (insulating) ceramics. The non-conducting (insulating) ceramics here may employ an oxide such as alumina, zirconia, or the like, or an insulating material of a high electric resistivity such as nitrogen silicon. In this case, when the respective rolling members (balls) 116 and 118 are formed of such non-conducting (insulating) ceramics, the material of the outer rings 108a and 110a and the inner rings 108b and 110b is not particularly limited, and high-carbon chrome bearing steel or special steel (stainless steel), for example, may be applied. [0075] Note that when the outer rings 108a and 110a are formed of such non-conducting (insulating) ceramics, the inner rings 108b and 110b and the rolling members (balls) 116 and 118 shouldbe formed of high-carbon chrome bearing steel or special steel (stainless steel) , for example. On the other hand, when the inner rings 108b and 110b are formed of such non-conducting (insulating) ceramics, the outer rings 108a and 110a and the rolling members (balls) 116 and 118 should be formed of high-carbon chrome bearing steel or special steel (stainless steel), for example. Moreover, use of grease for high speed bearings, for example, is preferably used as a lubricant for sealing the ! ball bearings 108 and 110. Note that the grease for high speed bearings may have ester oil, for example, added thereto as a base oil. [0076] Configuration Example 2: Case where conduction between housing and spindle is required> In the case where conduction between the housing 102 and the main shaft 104 is required, all of the outer rings 108a and 110a, the inner rings 108b and 110b, and the rolling members (balls) 116 and 118 should be made of conductive ceramics. The conductive ceramics here may employ a ceramic material of a low electric resistivity dispersed finely with conductive ceramic particles in an oxide, such as aluminum j oxide (alumina) or zirconium oxide (zirconia). j i [0077] In this case, use of conductive grease, for example, is preferably used as a lubricant for sealing the ball bearings 108 and 110. Moreover, the conductive grease may have carbon black, ametal powder, ametal oxide, or the like, added thereto as filler. Note that conduction indicates a state where electric current flows, namely a state capable of power distribution. [0077] According to this embodiment, the main shaft 104 may be supported sturdily in the housing 102 since the aforementioned ceramic ball bearings 108 and 110 have high bearing rigidity themselves . Therefore, the rotational axis L of the main shaft 104 may be kept constant without receiving any influence from a turning load of the turbine drive member 106 while the spindle device is operating, and the main shaft 104 may be rotated around the constant rotational axis L. As a result, for example, the main shaft 104 is never displaced so as to touch the housing 102 while the spindle device is operating. [0079] 5 In this case, since the rotational state (rotation speed) of the main shaft 104 may be kept constant, the rotation speed of the main shaft 104 may be stabilized at a constant ! desired speed. This allows uniform coating of an object to be coatedwithout anyunevenness on that object when the spindle .0 device is used as an electric painting device, for example. Moreover, while the spindle device needs to be enlarged ! since rigidity and load carrying capacity are determined by bearing size of the air bearings described above, use of the ceramic ball bearings 108 and 110 have high bearing rigidity .5 themselves instead of air bearings allows a compact spindle device. [0080] This allows significant reduction in cost for operating the spindle device than when the air bearings are applied. !0 Furthermore, compared to when the air bearings are applied, the number of components of the entire spindle device may be considerably reduced since the number of the ball bearings 108 and 110 can be decreased, and cost for manufacturing the spindle device may be significantly reduced as a result. 55 [0081] Yet further, since the ceramic ball bearings 108 and 110 may have greater rotating performance than the air bearings, demand for high-speed rotation (for example, high-speed rotation of 60,000 revolutions per minute (rpm)) required by the spindle device may be met. Note that the present invention is not limited to the above embodiments, and the technical ideas according to the following modifications are also contained within the technical scope of the present invention. [0082] For example, as illustrated in FIG. 11, the respective ball bearings 108 and 110 may be given a sealed structure in Configuration Examples 1 and 2 described above. In the drawing, as an example sealed structure, sealing plates 126, which seal divided internal spaces of the bearings between the outer rings 108a and 110a and the inner rings 108b and 110b from outside of the bearings, are provided to each of the ball bearings 108 and 110. [0083] A ring-shaped shield made by pressing a metal plate, for example, or a seal made of rubber containing a core bar maybe applied as the sealingplates 126 here. Note that while a structure applying the sealing plates 126, which have base ends fixed to the inner circumference of the outer rings 108a and 110a and front ends extending to the inner rings 108b and 110b, is illustrated as an example in the drawing, the reverse structure applying the sealing plates 126 having base ends fixed to the inner circumference of the inner rings 108b and 110 and front ends extending to the outer rings 10 8a and 110a is also possible. In this case, when seals are used as the sealing plates 126, the front ends of the seals 126 may be brought into contact with the other side raceway rings (namely, the outer rings 108 and 110a and the inner rings 108b and 110b), or small spaces may be kept without making contact therewith. [0084] According to this modification, in addition to the results according to the above embodiments, by application of the respective ball bearings 108 and 110 and the sealing plates 126, the lubricant (more specifically, the grease for high speed bearings in Configuration Example 1 and the conductive grease in Configuration Example 2) sealing the internal spaces of the ball bearings 108 and 110 leaking out of the bearings and scattering may be reliably inhibited. This allows a long operating life of the spindle device since the rotating performance and lubrication property of the ball bearings 108 and 110 may be kept constant over a long period of time. [0085] Alternatively, as illustrated in FIG. 10, for example, i a structure having at least the ball bearings 108 on one side be ceramic roller bearings is possible. Note that while as an example in the drawing, the ball bearings 108 are provided between the housing 102 and the main shaft 104 such that back surfaces 108d of the inner rings 108b are pressed against i the housing 102, this is not to limit the technical scope of the present invention. [0086] In this case, type of bearings on the other end side is not particularly limited; however, as an example in the drawing, air bearings are applied, having a structure including radial air bearings 128, which radially support the main shaft 104 in the housing 102, and axial air bearings . 130, which axially support the main shaft 104. [0087] The radial air bearings 128 include hollow cylinder-shaped porous members 12 8a, which are arranged concentrically with the rotational axis L so as to cover the ) periphery of the man shaft 104, and the axial air bearings 130 include circular porous members 130a, which are placed facing each other along one side (one side along the length of the rotational axis L) of the turbine impeller 106a of the turbine drive member 106. Moreover, a compressed air i channel 132 is established in the housing 102 for supplying compressed air to the porous members 128a and 130a, and a compressed air supply source not illustrated in the drawing is connected to the compressed air channel 132. [0088] ) According to such air bearings, if air current such as compressed air is supplied to the compressed air channel 132 from the compressed air supply source, that air current passes through the respective porous members 128a and 130a and blown to the periphery of the main shaft 104 and one side i of the turbine impeller 106a. At this time, a noncontact state is maintained between the main shaft 104 and the porous member 128a, and between the porous member 130a and one side of the turbine impeller 106a of the turbine drive member 106. [0089] Since the ball bearings 108 on one end may radially and axially support the main shaft 104 on its own, the porous member 130a of the axial air bearings 130 do not need to be 5 provided on either side so as to sandwich the turbine impeller 106a of the turbine drive member 106, where provision on only one side is sufficient. As a result, the main shaft 104 in its entirety including the turbine drive member 106 is supported by the ball bearings 108 on the one side in the 10 housing 102, and is also supported by the air bearings 128 j and 130 on the other end side floating above the housing 102. | [0090] According to this modification, in addition to the results according to the above embodiments, use of ceramic 15 roller bearings as the ball bearings 108 on the one side and use of only the bearings on the other end side as the air bearings 128 and 130 may considerably reduce the number of the air bearings 128 and 130. This allows significant reduction in cost for operating the spindle device since air 20 flow used for the air bearings 128 and 130 may be drastically decreased. Reference Signs List [0091] 2: main shaft 25 4: impeller 10: turbine blade 12: housing 14: bearing (radial static pressure gas bearing) 16: bearing (axial static pressure gas bearing) 28: nozzle (turbine air nozzle hole) 34: nozzle (brake air nozzle hole) Description of Embodiments 1. An air motor comprising: a housing, a main shaft inserted inside of the housing, an impeller fixed concentrically with the main shaft to an inserted portion of the main shaft inside of the housing and having a plurality of turbine blades formed on the outer periphery, bearings for rotatably supporting the main shaft and the impeller in the housing, and at least one nozzle having a tubular or hole-shaped channel for ejecting compressed air to the respective turbine blades for rotating the impeller along the circumference, wherein when Mi = ve/ a0 where rh denotes hydraulic radius of the channel of the nozzle, cf denotes viscous friction factor of a wall of the channel, k denotes specific heat ratio of compressed air, ve denotes flow velocity of the compressed air in an entrance of the channel, and ao denotes acoustic velocity, L is calculated using [Equation 1] L= ^r(isir+-2rM ^mO) (6) and the channel of the nozzle has a length set to a dimension of the calculated value L or greater. 2. The air motor of Claim 1, wherein the channel of the nozzle has a length set to a dimension of five times or more than the calculated value L or greater. 3. The air motor of Claim 1, wherein the bearings are static pressure gas bearings. 4. The air motor of Claim 1, wherein of the bearings, at least bearings on one end side are structured as ceramic roller bearings. 5. The air motor of Claim 4, wherein the roller bearings comprise a raceway ring on one side mounted on the housing, and a raceway ring on the other side mounted on a spindle facing the raceway ring on the one side, and a plurality of rolling elements incorporated between these bearing rings, and either the raceway ring or the rolling elements or all of them are made of ceramics. 6. The air motor of Claim 5, wherein either the raceway ring or the rolling elements or all of them are made of non-conducting ceramics. 7. The air motor of Claim 5, wherein the raceway ring and the rolling elements are made of non-conducting ceramics. 8. An electric painting device comprising the air motor of any one of Claims 1 to 7.