DESCRIPTION FUEL INJECTION VALVE Technical Field [0001] The present invention relates to an electromagnetic fuel injection valve used for an internal combustion engine or the like. Background Art [0002] A conventtonal fuel injection valve is configured to include a magnetic circuit including a cover, a casing, a mover, and a core. It is configured such that a magnetic. flux generated by a coil passes through a nonmagnetic sleeve from an inner periphery of the casing to enter a side surface of the mover and passes through an air gap from an end surface of the mover to enter an end surface of the core. In this configuration, the mover and the core are disposed in the nonmagnetic sleeve to generate an electromagnetic attraction force to the mover (for example, see Patent Document 1). Citation List Patent Literature [0003] Patent Document 1: JP-A-2005-282576 Summary of the Invention Technical Problem [0004] In the conventional fuel injection valve, as shown in Fig. 8, a mover 101 (armature) and a core 102 are opposed to each other with an air gap 103 interposed therebetween. The magnetic flux is generated in a magnetic path 103a passing through the air gap 103, and the electromagnetic attraction force is generated in a tension direction of the magnetic flux. However, among the products, there are variations in shape of the mover 101 and the core 102, and a variation in an inclination angle of a separation surface depending on arrangement of the mover 101 and the core 102 . Due to these variations, a magnetic resistance of the air gap 103 changes, and the magnetic flux generated in the magnetic path 103a passing through the air gap 103 changes. [0005] Further, in the conventional fuel injection valve, a sleeve 104 made of a nonmagnetic material is disposed on the outer peripheral side of the air gap 103 and is disposed so as to be inscribed in a casing 105 (housing). Therefore, a magnetic body serving as a magnetic path is not disposed on the outer peripheral side of the air gap 103. Therefore, a leakage magnetic flux is generated in a leakage magnetic path 104a bypassing the air gap 103 and extending from the side surface of the mover 101 to the side surface of the core 102. Since the leakage magnetic flux generated in the leakage magnetic path 104a has a component extending obliquely from an outer peripheral surface of the mover 101, it affects the electromagnetic attraction force. [0006] The fuel injection valve having such a conventional structure has a change in the magnetic resistance in the air gap depending on the variations in the shape of the mover 101 and the core 102 and the variation in the inclination angle therebetween. Therefore, the magnetic fluxes passing through the magnetic path 103a and the leakage magnetic path 104a also vary, and there has been a problem that a variation in the electromagnetic attraction force among the products is large. [0007] Here, the fuel injection valve, which is supplied with a battery voltage and supplies a current to the coil, must ensure a valve opening operation even when the battery voltage is low at a low temperature start of an internal combustion engine. In a conventional general fuel injection valve, at a low temperature initial stage of an operation of the internal combustion engine, for example, when a coil resistance is 12 Q. and the battery voltage is 6 V, a small current of about 0.5 A is supplied. In the initial stage of the operation, since a needle integrated with the mover 101 is in a valve closed state and the air gap 103 between the mover 101 and the core 102 is large, a relatively small electromagnetic attraction force is generated. On the other hand, in a steady state of the internal combustion engine, since a voltage of about 12 V is supplied from a generator, a large current of about 1 A is supplied. Since the needle is in a valve open state and the air gap 103 is small, a relatively large electromagnetic attraction force is generated. [0008] In order to guarantee the operation at low temperature, when an opposing area of the mover 101 is increased in an attempt to secure the electromagnetic attraction force, there is a problem that the electromagnetic attraction force in the steady state is excessive, a response delay occurs at a transition from the valve open state to the valve closing state, and accuracy of a minute injection amount is reduced. [0009] As described above, when the variation in the electromagnetic attraction force of the fuel injection valve among the products is large, there has been a problem that a fuel injection amount changes and accuracy of air-fuel ratio in the internal combustion engine is reduced. In addition, when the response delay at the time of valve closing is large, there has been a problem that the variation of the minute injection amount among the products is large, and the accuracy of the air-fuel ratio at a light load in the internal combustion engine is reduced. [0010] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to improve the accuracy of the injection amount -by preventing the variation of the electromagnetic attraction force of the fuel injection valve. Means for Solving the Problems [0011] A fuel injection valve according to the present invention is a fuel injection valve including an armature provided integrally with a valve body of a magnetic fuel injection valve and coming into contact with or separated from a core by an electromagnetic attraction force, and a holder made of a magnetic material provided on the outer peripheral side of the core and the armature. An electromagnetic path extending from the holder to the core is formed of a first magnetic path extending from the holder through the armature to the core, a second magnetic path extending from the holder directly to the core, and a common magnetic path extending to the first magnetic path and the second magnetic path. A magnetic throttle portion for adjusting a magnetic flux passing through the first magnetic path is provided in the common magnetic path of the holder. A magnetic path cross-sectional area of the magnetic throttle portion is smaller than a total area of a minimum magnetic path cross-sectional area of the first magnetic path and a minimum magnetic path cross-sectional area of the second magnetic path. [0012] Further, a fuel injection valve according to the present invention is a fuel injection valve including an armature provided integrally with a valve body of a magnetic fuel injection valve and coming into contact with or separated from a core by an electromagnetic attraction force, a holder made of a magnetic material provided on the outer peripheral side of the core and the armature, and a housing provided on the outer peripheral side of the holder. An electromagnetic path extending from the housing through the holder to the core is formed of a first magnetic path extending from the holder through the armature to the core, a second magnetic path extending from the holder directly to the core, and a common magnetic path extending to the first magnetic path and the second magnetic path. A magnetic throttle portion for adjusting a magnetic flux passing through the first magnetic path is provided in the common magnetic path of the housing. A magnetic path cross-sectional area of the magnetic throttle portion is smaller than a total area of a minimum magnetic path cross-sectional area of the first magnetic path and a minimum magnetic path cross-sectional area of the second magnetic path. Advantage of the Invention [0013] In the fuel injection valve according to the present invention, a holder made of a magnetic material serving as a magnetic circuit forming portion is disposed on the outer peripheral side of the core and the armature. Further, a magnetic throttle portion is formed in the common magnetic path extending to the first magnetic path and the second magnetic path of the holder or the housing. By managing the cross-sectional area of the magnetic throttle portion and adjusting the magnetic flux passing through the first magnetic path, it is possible to prevent the variation in the electromagnetic attraction force among the products. Other objects, features, aspects and effects of the present invention will become more apparent from the following detailed description of the present invention with reference to the drawings. Brief Description of the Drawings [0014] [Fig. 1] Fig. 1 is a side cross-sectional view of a fuel injection valve according to Embodiment 1 of the present invention. [Figs. 2A, 2B] Figs. 2A, 2B are cross-sectional views of a main part of a magnetic circuit forming portion at the time of valve opening of the fuel injection valve according to Embodiment 1, Fig. 2A shows a magnetic path, and Fig. 2B shows a magnetic path cross-sectional area in a magnetic path main part. [Figs. 3A, 3B] Figs. 3A, 3B are diagrams necessary for describing Embodiment 1, Fig. 3A shows a magnetic circuit of a fuel injection valve as Comparative Example, and Fig. 3B shows a magnetic circuit of the fuel injection valve according to Embodiment 1. [Fig. 4] Fig. 4 is a diagram showing magnetomotive force dependence of a magnetic flux of the fuel injection valve according to Embodiment 1. [Fig. 5] Fig. 5 is a side cross-sectional view of a fuel injection valve according to Embodiment 2 of the present invention. [Figs. 6A, 6B] Figs. 6A, 6B are cross-sectional views of the main part of the magnetic circuit forming portion of the fuel injection valve according to Embodiment 2, Fig. 6A shows a state when the valve is open, and Fig. 6B shows a state when the valve is closed. [Fig. 7] Fig. 7 is a side cross-sectional view of a fuel injection valve according to Embodiment 3 of the present invention. [Fig. 8] Fig. 8 is a cross-sectional view of a magnetic circuit forming portion of a conventional fuel injection valve. Mode for Carrying Out the Invention [0015] Embodiment 1 Configuration of a fuel injection valve 100 according to Embodiment 1 of the present invention will be described with reference to Figs. 1 to 4. Fig. 1 is a side cross-sectional view showing the entire structure of the fuel injection valve 100 according to Embodiment 1 of the present invention. As shown in Fig. 1, an outer shape of the fuel injection valve 100 is formed by a resin molded part 1, and a fuel path is formed in a direction from the top to the bottom of the paper. A valve seat 2 having a conical seat surface 2a is disposed in the vicinity of an injection port which is a lower portion of the fuel injection valve 100. A fuel injection amount is controlled by a separation state of a valve body 3 from the seat surface 2a. [0016] The valve body 3 is fixed to an armature 5 (mover) movable by electromagnetic control through a needle 4. An armature end surface 5a is formed in the armature 5 on the upstream side of the fuel path. A core 6 serving as a base portion of a fuel flow path (pipe) is disposed on the upstream side of the fuel path with respect to the armature 5. A core end surface 6a opposed to the armature end surface 5a is formed on a downstream side end surface of the core 6. [0017] A gap between contact separation surfaces of the armature 5 and the core 6 is an air gap. A holder 7 made of a magnetic material is disposed on the outer peripheral side of the contact separation surface between the armature 5 and the core 6. The holder 7 is a component part of a magnetic circuit. The core 6 is press-fitted into the holder 7, and both are joined to each other. The valve seat 2 is held on the downstream side of the fuel path inside the holder 7. [0018] Further, a housing 8 which is a component part of the magnetic circuit is disposed on the outer peripheral side of the holder 7. A coil 9 is disposed between the housing 8 and the core 6, and the housing 8 and the core 6 are joined via a cover 10. A rod 11 is fixed to an inner diameter of the core 6 and a spring 12 for applying an axial load to the needle 4 is disposed between the rod 11 and the needle 4. [0019] The fuel injection valve 100 is connected in a fuel sealed state to a fuel pipe on the upstream side via a rubber ring 13. The fuel is supplied into the fuel injection valve 100 ' through a filter 14 disposed at a fuel supply end of the core 6. The supplied fuel passes through the fuel flow path of each part and reaches a contact portion between the valve body 3 and the seat surface 2a, and when it is in a valve open state, the fuel is injected through an orifice 15a of an orifice plate 15. [0020] Next, a valve opening operation at an initial stage of an operation of the fuel injection valve 100 will be described. In the fuel injection valve 100, when a voltage between two terminals 16 (only one terminal is shown in Fig. 1) is turned on, a current flows through the coil 9 connected to the terminal 16. Then, due to a magnetic field generated by energization, a magnetic flux circulates in the magnetic circuit mainly including the magnetic material of the core 6, the cover 10, the housing 8, the holder 7 and the armature 5. [0021] The magnetic flux passes between the armature 5 and the core 6 in a direction substantially perpendicular to the armature end surface 5a and the core end surface 6a. Then, a tension is applied in a direction of the magnetic flux, and an electromagnetic attraction force to the core 6 is generated in the armature 5 which is a movable part. When the electromagnetic attraction force exceeds a load of the spring 12, the needle 4 starts to be displaced. The displacement of the needle 4 is completed at a position where the armature end surface 5a contacts the core end surface 6a, to be in the valve open state, and a gap of about 40 to 60 |im is formed between the valve body 3 and the seat surface 2a. The fuel passes through this gap and is injected to an outside from the orifice 15a. [0022] Next, a valve closing operation of the fuel injection valve 100 will be described. When the voltage between the two terminals 16 is turned off, the current of the coil 9 is interrupted. When the current is interrupted, the magnetic flux decreases and the electromagnetic attraction force acting on the armature 5 also decreases, and when the electromagnetic attraction force falls below the load of the spring 12 after a predetermined time lapses, the needle 4 starts to be displaced. A displacement profile of the needle 4 with respect to a time after the voltage is turned off is displaced in a parabolic shape after a predetermined time without displacement lapses, and the displacement is completed when the valve body 3 comes into contact with the seat surface 2a. As the displacement is completed, the fuel injection is terminated. [0023] Next, Fig. 2A and Fig. 2B are cross-sectional views of a main part of a magnetic circuit forming portion at the time of valve opening, and the magnetic path and its cross-sectional area will be described. Fig. 2A shows a magnetic flux flow (an electromagnetic path) in the vicinity of the air gap between _ the armature end surface 5a and the core end surface 6a in the fuel injection valve 100, and Fig. 2B shows a magnetic path cross-sectional area of each main part of the magnetic circuit. [0024] As shown in Fig. 2A, the magnetic flux in the housing 8 reaches the core 6, and the electromagnetic path includes a common magnetic path C passing through an axial direction of the holder 7 from a press-fitting portion (joining portion) of the housing 8 and the holder 7 to a lower end of the armature 5, a magnetic path A (corresponding to a first magnetic path) branching from the common magnetic path C and extending to the core 6 through the armature 5, and a magnetic path B (corresponding to a second magnetic path) extending from the holder 7 directly to the core 6. In Fig. 2A, the magnetic path A is indicated by a short dashed line, the magnetic path B is indicated by a long dashed line, and the common magnetic path C is indicated by a hollow line. [0025] Fig. 2B is a cross-sectional view of the armature 5, the core 6, and the holder 7 taken out from Fig. 2A. In Fig. 2B, a plurality of constituting portions having different magnetic path cross-sectional areas of the holder 7 are formed, and these constituting portions are arranged and formed in the order of a first cylindrical portion 7a, a second cylindrical portion 7b, a third cylindrical portion 7c, and a fourth cylindrical portion 7d from the downstream side to the upstream side of the flow path. [0026] The first cylindrical portion 7a of the holder 7 is a thick portion press-fitted into the housing 8, and is a portion to be the common magnetic path C. Subsequently, the second cylindrical portion 7b formed in the axial direction is a portion to be the common magnetic path C extending from the first cylindrical portion 7a to a lower end of a junction between the holder 7 and the armature 5. This second cylindrical portion 7b corresponds to a magnetic throttle portion which is a feature of the present invention. A cross-sectional area of the second cylindrical portion 7b serving as the magnetic throttle portion is formed so as to be minimized in the magnetic circuit of the fuel injection valve 100. Thai is, the cross-sectional area of the second cylindrical portion 7b corresponds to a magnetic path cross-sectional area S2 of the magnetic throttle portion. [0027] The second cylindrical portion 7b can be formed, for example, by cutting the holder 7, and it is possible to secure accuracy of magnetic flux adjustment of the magnetic throttle portion by achieving an arbitrary dimensional accuracy. [0028] Subsequently, the third cylindrical portion 7c is a portion formed on the outer peripheral side of the air gap which is the gap between the contact separation surfaces of the armature 5 and the core 6, and to be the magnetic path B. The cross-sectional area of the third cylindrical portion 7c is the smallest in the holder 7 and corresponds to a magnetic path cross-sectional area SB which is the minimum in the magnetic path B. Subsequently, the fourth cylindrical portion 7d is a portion press-fitted into the core 6. [0029] Here, a magnetic path cross-sectional area of a joint surface between the armature 5 and the core 6 is expressed as SA. The magnetic circuit of the fuel injection valve 100 according to the present invention is formed to have dimensions satisfying SA + SB = Si, Si> S2. [0030] Next, the magnetic flux passing through each magnetic path will be described. As shown in Fig. 2A, since an outer periphery of the holder 7 is press-fitted into and in contact with an inner periphery of the housing 8 in the common magnetic path C, the magnetic flux penetrates from the housing 8 to the first cylindrical portion 7a with almost no magnetic loss. When the magnetic flux passing through the first cylindrical portion 7a is equal to or larger than a value corresponding to the magnetic path cross-sectional area of the second cylindrical portion 7b, the magnetic throttle portion of the second cylindrical portion 7b is magnetically saturated, so that the magnetic flux passing axially upwardly through the second cylindrical portion 7b is adjusted (restricted) so as to have a predetermined value. [0031] The magnetic flux adjusted at the magnetic throttle portion of the holder 7 branches into the magnetic path A and the magnetic path B and proceeds upwardly in the axial direction. At this time, since the third cylindrical portion 7c constituting the magnetic path B is formed to have the smallest cross-sectional area in the magnetic circuit, the third cylindrical portion 7c is in a saturated state, and the remaining magnetic flux not flowing in the magnetic path B proceeds to the magnetic path A. The magnetic path A and the magnetic path B extend to the core 6 to be joined together. [0032] Further, the magnetic circuit of the fuel injection valve 100 will be described in detail. In a steady operation state of an internal combustion engine, when a current of 1 A is supplied to the coil of 340 turns, a comparatively large magnetomotive force of 340A is generated in the magnetic circuit, and the second cylindrical portion 7b of the holder 7 corresponding to the magnetic .throttle portion is in a magnetic saturation state. The number of magnetic fluxes circulating in the magnetic circuit is limited to the number of saturation magnetic fluxes of the magnetic throttle portion (second cylindrical portion 7b) of the holder 7 which is the minimum magnetic path cross-sectional area portion of the common magnetic path C. [0033] A part of the magnetic flux of the common magnetic path C branches to reach the core 6 through the magnetic path B. That is, the part of the magnetic flux passing through the second cylindrical portion 7b of the holder 7 enters the core 6 through the third cylindrical portion 7c and the fourth cylindrical portion 7d. Then, the remaining magnetic flux branched from the common magnetic path C extends to the core 6 through the magnetic path A. That is, the remaining magnetic flux, which has passed through the second cylindrical portion 7b of the holder 7 and has not proceeded to the magnetic path B, enters an outer peripheral surface of the armature 5 from an inner peripheral surface of the second cylindrical portion 7b, passes in the axial direction inside the armature 5, and enters the core 6 from the core end surface 6a through the air gap from the armature end surface 5a. [0034] Here, for example, the magnetic path cross-sectional area SB which is the minimum in the magnetic path B is a portion where a wall thickness of the third cylindrical portion 7c of the holder 7 is about 0.2 mm. The magnetic path cross-sectional area SB is formed to be 1/3 or less of the magnetic path cross-sectional area S2 of the magnetic throttle portion of the second cylindrical portion 7b. The third cylindrical portion 7c of the holder 7 is magnetically saturated like the magnetic throttle portion. Therefore, it is possible to adjust the magnetic flux passing through the magnetic path A by the magnetic throttle portion. [0035] On the other hand, the magnetic path cross-sectional area SA which is the minimum in the magnetic path A corresponds to an area of the end surfaces of the armature 5 and the core 6 in the air gap portion. A magnetic path cross-sectional area Si which is a total of the magnetic path A and the magnetic path B in the air gap portion has a relationship of Si = SA + SB > S2 because S2 is the minimum cross-sectional area in the magnetic circuit. [0036] Here, for example, by controlling the dimensions so that Si:S2 * 10:9, an effect of preventing the electromagnetic attraction force from variation becomes prominent. Compared with the above relationship, when S2 is smaller, the electromagnetic attraction force is insufficient, and when S2 is larger and approaches a value of Si too much, it is reversed by a dimensional tolerance and the effect of preventing the electromagnetic attraction force from variation is reduced in some cases. [0037] Next, with reference to Figs. 3A and 3B, the magnetic circuits are compared between a fuel injection valve of a conventional structure as Comparative Example and the fuel injection valve 100 according to the present invention of the present application. In Fig. 3A and Fig. 3B, Fig. 3A is a circuit diagram showing a magnetic circuit of the conventional fuel injection valve as Comparative Example, and Fig. 3B is a circuit diagram showing a magnetic circuit of the fuel injection valve 100 of Embodiment 1. The circuit diagram of Fig. 3A reflects the structure of the conventional fuel injection valve shown in Fig. 8, and it is assumed that a magnetic path 103a from a mover 101 (armature) to a core 102 in Fig. 8 corresponds to the magnetic path A in Fig. 2A, and a leakage magnetic path 104a generated in a sleeve 104 correspond to the magnetic path B in Fig. 2B. The leakage magnetic path 104a is denoted by a symbol B0 in Fig. 3A. [0038] In the circuit diagrams of Fig. 3A and Fig. 3B, the magnetomotive force of the coil is denoted by E, a magnetic resistance of the magnetic path A is denoted by RA, and a magnetic resistance of the leakage magnetic path B0 is denoted by RB0. The electromotive force E of the magnetic circuit corresponds to a DC voltage of an electric circuit, and the magnetic resistances RA and RB0 correspond to electric resistances. Further, the magnetic flux flowing in the magnetic circuit corresponds to the current. [0039] As shown in Fig. 3A, in the conventional magnetic circuit, the leakage magnetic path B0 is generated in parallel with the magnetic path A, a magnetic flux OA passing through the magnetic path A and a magnetic flux OB0 passing through the leakage magnetic path B0 vary depending on the magnetomotive force E of the coil and the magnetic resistances RA and RB0, and both