.FORM 2 THE PATENT ACT 197 0 (39 of 1970) & The Patents Rules, 2003 COMPLETE SPECIFICATION (See Section 10, and rule 13) 1. TITLE OF INVENTION MACHINE DISPLACEMENT ADJUSTMENT SYSTEM FOR MACHINE TOOLS 2. APPLICANT(S) a) Name : MITSUBISHI HEAVY INDUSTRIES, LTD. b) Nationality : JAPANESE Company c) Address : 16-5, KONAN 2-CHOME, MINATO-KU, TOKYO 1088215, JAPAN 3. PREAMBLE TO THE DESCRIPTION The following specification particularly describes the invention and. the manner in which it is to be performed : - TECHNICAL FIELD The present invention relates to a machine displacement correction system for correcting machine displacement (thermal displacement, self-weight displacement, level displacement) of a machine tool. BACKGROUND ART In general, a full closed-loop feedback control system as shown in FIG. 7 is employed in a servo control device performing positioning control of a machine tool. Although a specific description is omitted herein, the servo control device shown in FIG. 7 performs positioning control in a way that makes the position of a moving body 1 follow a position command by controlling the rotation of a servomotor 3 on the basis of position feedback information (i.e., machine end position information) from a position detector 2, which is provided to the moving body 1, and velocity feedback information to be fed back via a differential operation unit 5 from a pulse coder 4, which is provided to the servomotor 3. Incidentally, Kp denotes the position loop gain, Kv denotes the velocity loop proportional gain, Kvi denotes the velocity loop integral gain and s denotes the Laplace operator in FIG. 7. As described above, the full closed-loop feedback control system uses the machine end position information as the position feedback information. However, when machine displacement occurs in each structure of a machine tool due to a temperature change in heat sources such as a main spindle and the servomotor 3 included in the machine tool, and due to a temperature change in the outside air, static accuracy such as positioning accuracy of each moving axis in the machine tool or positioning accuracy of a tool in a three-dimensional space is degraded. The machine displacement occurs not only due to thermal displacement, but also due to deflection caused by self-weight, deflection of a structure caused by level displacement, and the like. Moreover, in a case where a semi closed-loop feedback control system as shown in FIG. 8 is employed as a control system of a machine tool, the static accuracy tends to degrade even more because position information on the servomotor 3 (a rotation angle of the servomotor 3 that is detected by the pulse coder 4) is used as the position feedback information. Note that such machine displacement occurs similarly in control of a robot or the like. The degradation in the static accuracy due to such machine displacement, particularly, the degradation in the static accuracy due to machine displacement occurring due to heat or the like is a large factor in an increase in machining error, and is still a large problem today. As a measure for the degradation in the static accuracy, provision of a thermal displacement correction system to a control system for a machine tool has been known. With a temperature sensor embedded in the machine, the thermal displacement correction system estimates a thermal displacement amount of the machine by using a simple arithmetic expression on the basis of the temperature data, and thus compensates the machine displacement amount by shifting a machine coordinate or the like by the displacement amount. A specific example of the thermal displacement correction system is shown in FIG. 9 and FIG. 10. FIG. 9 shows a case of a horizontal machining center. Here, temperature sensors 23-1 to 23-10 are installed to a bed 11, a column 12, a saddle 13 movable in an X-axis direction, a head 14 provided with a main spindle 25 and movable in a Z-axis direction, a table 15 movable in a Y-axis direction and a workpiece W, which is placed on the table 15. The temperature sensors 23-1 to 23-10 detect the temperatures of the corresponding structures (the bed 11, the column 12, the saddle 13, the head 14, and the table 15) and the workpiece W, and then output temperature data sets (temperature detection signals) a1 to a10. A correction device 24 includes a temperature data receiving unit 16, a thermal displacement amount calculating unit 17 and a correction amount calculating unit 18. The temperature data receiving unit 16 receives the temperature data sets a1 to a10 from the temperature sensors 23-1 to 23-10. The thermal displacement amount calculating unit 17 calculates the displacement amounts of the structures (the bed 11, the column 12, the saddle 13, the head 14 and the table 15) and the workpiece W due to heat on the basis of the temperature data sets a1 to a10 received by the temperature data receiving unit 16. The correction amount calculating unit 18 calculates the displacement amounts of the moving axes (the X-axis, the Y-axis and the Z-axis) on the basis of the thermal displacement amounts of the structures (the bed 11, the column 12, the saddle 13, the head 14 and the table 15) and the workpiece W calculated by the thermal displacement amount calculating unit 17. The correction amount calculating unit 18 then sets the values of these displacement amounts with a reversed sign as the correction amounts of the moving axes (the X-axis, the Y-axis and the Z-axis), and sends the correction amounts to servo control devices 19, 20, 21 of the respective moving axes (the X-axis, the Y-axis and the Z-axis). In the servo control device 19 of the X-axis, a deviation operation unit 22 corrects an X-axis position command by adding the correction amount of the X-axis (= "a minus displacement amount of the X-axis") calculated by the correction amount calculating unit 18 to the X-axis position command, and performs arithmetic on a deviation between the corrected X-axis position command and X-axis position feedback information. In the servo control device 20 of the Y-axis, a deviation operation unit 22 corrects a Y-axis position command by adding the correction amount of the Y-axis (= "a minus displacement amount of the Y-axis") calculated by the correction amount calculating unit 18 to the Y-axis position command, and performs arithmetic on a deviation between the corrected Y-axis position command and Y-axis position feedback information. In the servo control device 21 of the Z-axis, a deviation operation unit 22 corrects a Z-axis position command by adding the correction amount of the Z-axis (= "a minus displacement amount of the Z-axis") calculated by the correction amount calculating unit 18 to the Z-axis position command, and performs arithmetic on a deviation between the corrected Z-axis position command and Z-axis position feedback information. FIG. 10 shows a case of a portal machining center. Here, temperature sensors 45-1 to 45-8 are installed to a bed 31, a gate-shaped column 32, a ram 35 in which a main spindle 36 is incorporated, a table 37, and a workpiece W, which is placed on the table 37. The temperature sensors 45-1 to 45-8 detect the temperatures of the corresponding structures (the bed 31, the column 32, the ram 35 and the table 37) and the workpiece W, and then output temperature data sets (temperature detection signals) b1 to b8. Note that the table 37 is movable in an X-axis direction, a saddle 34 is movable in a Y-axis direction along a cross rail 33, and the ram 35 (the main spindle 36) is movable in a Z-axis direction. A correction device 46 includes a temperature data receiving unit 38, a thermal displacement amount calculating unit 39 and a correction amount calculating unit 40. The temperature data receiving unit 38 receives the temperature data sets b1 to b8 from the temperature sensors 45-1 to 45-8. The thermal displacement amount calculating unit 39 calculates the displacement amounts of the structures (the bed 31, the column 32, the ram 35 and the table 37) and the workpiece W due to heat on the basis of the temperature data sets b1 to b8 received by the temperature data receiving unit 38. The correction amount calculating unit 40 calculates the displacement amounts of the moving axes (the X-axis, the Y-axis and the Z-axis) on the basis of the thermal displacement amounts of the structures (the bed 31, the column 32, the ram 35 and the table 37) and the workpiece W calculated by the thermal displacement amount calculating unit 39. The correction amount calculating unit 40 then sets the values of these displacement amounts with a reversed sign as the correction amounts of the moving axes (the X-axis, the Y-axis and the Z-axis), and sends the correction amounts to servo control devices 41, 42 and 43 of the respective moving axes (X-axis, Y-axis and Z-axis). In the servo control device 41 of the X-axis, a deviation operation unit 44 corrects an X-axis position command by adding the correction amount of the X-axis (= "a minus displacement amount of the X-axis") calculated by the correction amount calculating unit 40 to the X-axis position command, and performs arithmetic on a deviation between the corrected X-axis position command and X-axis position feedback information. In the servo control device 42 of the Y-axis, a deviation operation unit 44 corrects a Y-axis position command by adding the correction amount of the Y-axis (= "a minus displacement amount of the Y-axis") calculated by the correction amount calculating unit 40 to the Y-axis position command, and performs arithmetic on a deviation between the corrected Y-axis position command and Y-axis position feedback information. In the servo control device 43 of the Z-axis, a deviation operation unit 44 corrects a Z-axis position command by adding the correction amount of the Z-axis (= "a minus displacement amount of the Z-axis") calculated by the correction amount calculating unit 40 to the Z-axis position command, and performs arithmetic on a deviation between the corrected Z-axis position command and Z-axis position feedback information. Patent Documents 1 to 5 given below can be cited as prior art documents related to the above-described thermal displacement correction system using the temperature sensors. PRIOR ART DOCUMENT PATENT DOCUMENTS Patent Document 1: Japanese Patent Application Publication No. Hei 10-6183 Patent Document 2: Japanese Patent Application Publication No. 2006-281420 Patent Document 3: Japanese Patent Application Publication No. 2006-15461 Patent Document 4: Japanese Patent Application Publication No. 2007-15094 Patent Document 5: Japanese Patent Application Publication No. 2008-183653 Patent Document 6: Japanese Patent Application Publication No. 2007-175818 Patent Document 7: Japanese Patent Application Publication No. Hei 11-226846 SUMMARY OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION The number of the temperature sensors used for the estimation of the thermal displacement amount of a machine is limited, however. Thus, it is difficult to completely figure out the thermal displacement amount of the machine. In addition, because the conventional method finds the thermal displacement mode and the thermal displacement amount of the machine by estimation from the detection values of the temperature sensors, the thermal displacement cannot be completely compensated. Meanwhile, for the purpose of setting the thermal displacement of a machine as a straightforward thermal displacement mode as much as possible, the invention as recited in Patent Document 6 given above or the like has been proposed. However, it is difficult to set the thermal displacement of the machine that is caused by a change in the temperature of the outside air as a completely straightforwardthermal displacement mode (i.e., as only a stretch and contraction mode eliminating warpage, tilt or the like of a column or the like). It is difficult to completely eliminate the warpage or tilt of the column or the like that is caused by a change in the temperature of the outside air or the like. Accordingly, the present invention has been made in view of the aforementioned circumstance, and aims to provide a machine displacement correction system for machine tools that uses a tilt angle detector such as a level capable of directly detecting a tilt angle of a structure of a machine, such as a column. Note that although an invention that uses levels is proposed in Patent Document 7 given above, this invention relates to a posture control system that uses levels and piezoelectric actuators in combination: Accordingly, this system is not a system configured to correct machine displacement, and deviates from the object of the present invention. MEANS FOR SOLVING THE PROBLEMS A machine displacement correction system for a machine tool of a first invention for solving the foregoing problem is a machine displacement correction system configured to correct machine displacement of a machine tool. The system comprises: a tilt angle detector installed to a structure of the machine tool, and configured to detect a tilt angle of the structure and to output tilt amount data; and a correction device including: a tilt amount data receiving unit configured to receive the tilt amount data from the tilt angle detector; a machine displacement amount calculating unit configured to calculate a machine displacement amount of the structure on the basis of the tilt amount data received by the tilt amount data receiving unit; and a correction amount calculating unit configured to calculate a correction amount of a moving axis of the machine tool on the basis of the machine displacement amount of the structure calculated by the machine displacement amount calculating unit. In addition, a machine displacement correction system for a machine tool of a second invention is a machine displacement correction system configured to correct machine displacement of a machine tool. The system comprises: a tilt angle detector installed to a structure of the machine tool, and configured to detect a tilt angle of the structure and to output tilt amount data; a temperature sensor installed to a structure of the machine tool or a workpiece, and configured to detect a temperature of the structure or the workpiece and to output temperature data; and a correction device including: a tilt amount data receiving unit configured to receive the tilt amount data from the tilt angle detector; a machine displacement amount calculating unit configured to calculate a machine displacement amount of the structure on the basis of the tilt amount data received by the tilt amount data receiving unit; a first correction amount calculating unit configured to calculate a first correction amount of a moving axis of the machine tool on the basis of the machine displacement amount of the structure calculated by the machine displacement amount calculating unit; a temperature data receiving unit configured to receive the temperature data from the temperature sensor; a thermal displacement amount calculating unit configured to calculate a thermal displacement amount of the structure or the workpiece on the basis of the temperature data received by the temperature data receiving unit; a second correction amount calculating unit configured to calculate a second correction amount of the moving axis on the basis of the thermal displacement amount of the structure or the workpiece calculated by the thermal displacement amount calculating unit; and a correction amount adder configured to add the first correction amount calculated by the first correction amount calculating unit and the second correction amount calculated by the second correction amount calculating unit. EFFECTS OF THE INVENTION The machine displacement correction system for a machine too! according to the first invention is capable of directly figuring out the tilt amount (tilt angle) of the structure of the machine tool with the tilt angle detector (a level, for example) when the structure of the machine tool is inclined due to machine displacement such as warpage and tilt (thermal displacement, self-weight displacement, or level displacement, or a mixture of thermal displacement, self-weight displacement and level displacement). Thus, the machine displacement correction system is capable of estimating the machine displacement amount of the structure with high accuracy by calculating the machine displacement amount of the structure on the basis of the tilt amount data on the structure that is directly figured out by the tilt angle detector. Accordingly, the machine displacement correction system is capable of obtaining the correction amount of the moving axis with high accuracy on the basis of the machine displacement amount. For this reason, a highly-accurate compensation system can be realized. As in the case of the first invention, the machine displacement correction system for a machine tool according to the second invention is capable of directly figuring out the tilt amount (tilt angle) of the structure of the machine tool with the tilt angle detector (a level, for example) when the structure of the machine tool is inclined due to machine displacement such as warpage and tilt (thermal displacement, self-weight displacement, or level displacement, or a mixture of thermal displacement, self-weight displacement and level displacement). Thus, the machine displacement correction system is capable of estimating the machine displacement amount of the structure with high accuracy by calculating the machine displacement amount of the structure on the basis of the tilt amount data on the structure that is directly figured out by the tilt angle detector. Accordingly, the machine displacement correction system is capable of obtaining the first correction amount of the moving axis with high accuracy on the basis of the machine displacement amount. Moreover, the second invention makes it possible to deal with not only the machine displacement such as warpage and tilt, but also the thermal displacement such as stretch of the structure and stretch of the workpiece due to heat, because the second correction amount of the moving axis that is found on the basis of the temperature data from the temperature sensor is added to the first correction amount of the moving axis. Accordingly, the second invention makes it possible to obtain the correction amount of the moving axis with higher accuracy. Thus, the second invention can realize a highly-accurate compensation system. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram relating to a machine displacement correction system using levels according to Embodiment 1 of the present invention, and is a perspective view of a machine tool (portal machining center) showing the arrangement of the levels. FIG. 2 is a diagram relating to the machine displacement correction system using the levels according to Embodiment 1 of the present invention, and is the diagram showing a configuration of a correction device. FIG. 3 is a diagram showing an example of calculating an amount of machine displacement attributable to a tilt. FIG. 4 is a diagram relating to a machine displacement correction system using levels according to Embodiment 2 of the present invention, and is a perspective view of a machine tool (portal machining center) showing the arrangement of the levels. FIG. 5 is a diagram relating to the machine displacement correction system using the levels according to Embodiment 2 of the present invention, and is the diagram showing a configuration of a correction device. FIG. 6 is a diagram showing an example of calculating an amount of thermal displacement attributable to a temperature change. FIG. 7 is a block diagram showing a configuration of a full closed-loop servo control device (feedback control system). FIG. 8 is a block diagram showing a configuration of a semi closed-loop servo control device (feedback control system). FIG. 9 is a diagram showing a configuration example of a conventional thermal displacement correction system using temperature sensors. FIG. 10 is a diagram showing another configuration example of the conventional thermal displacement correction system using temperature sensors. MODES FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail on the basis of the drawings.