DESCRIPTION FUEL INJECTION VALVE Technical Field [0001] The present invention relates to a fuel injection valve used in an engine and, more specifically, to a fuel injection valve in which atomization of fuel spray is improved while restraining excessive spray diffusion. Background Art [0002] Recently, improvement of flexibility of direction of fuel spray injection injected from a fuel injection valve and atomization of fuel spray is required in a circumstance in which exhaust gas regulation for automotive vehicles or the like is tightened. In particular, atomization of fuel spray has been studied in various manner, and, for example, disclosed in Patent Publication No. 3183156 (hereinafter, referred to as Patent Document 1) is a fluid injection nozzle in which injection holes are arranged inside an imaginary envelope along an intersection between the main flow direction of fluid on the downstream side of a seat portion of a valve seat and an injection hole plate, a portion of the distal end of a valve element, which is located inwardly of the seat portion and opposes the injection holes is formed into a flat plane parallel to the injection hole plate^ the injection holes are inclined with respect to the injection hole plate by a predetermined angle and satisfy a relation h<1.5d where d represents the diameter of the injection hole, and h represents the vertical distance between the flat plane of the valve element and the injection hole plate with the valve opened. According to this injection nozzle, after fuel is flowed out from the seat portion of the valve seat, the flow is converted to a flow along the injection hole plate in a cavity interposed between the flat plane of the valve element and the injection hole plate, and a flow proceeding directly to the injection holes and a flow passing between the injection holes, making a U-turn at the center of the injection hole plate by the opposing flow and proceeding to the injection holes are generated, whereby flows which proceed uniformly toward the injection holes may be obtained. Accordingly, collision between the fuel flows may be induced immediately above the injection holes, whereby atomization is accelerated. [0003] Patent Document 1: Japanese Patent No. 3183156 Disclosure of Invention Problems to be Solved by the Invention [0004] In the technology disclosed in Patent Document 1, conversion of the flow of fuel flowed out from the seat portion of the valve seat into the flow parallel to the injection hole plate in the cavity interposed between the flat plane of the valve element and the injection hole plate is used for atomization of fuel spray. However, the flow rate of the fuel entering into the injection holes is subjected to variation in height of the cavity. Therefore, since the height of the cavity defined in Patent Document 1 is affected by variation in machining or the inclination of the valve element with the valve opened, it has a disadvantage such that flow rate accuracy or spray characteristic tend to vary from one fuel injection valve to another. [0005] Means to cause the flow toward the center of the injection hole plate and the flow passing between the injection holes, making the U-turn at the center of the injection hole plate by the opposing flow and proceeding to the injection holes to collide with each other immediately above the injection holes is effective for atomization. However, the spray Lends to diffuse easily, and hence directivity of spray is deteriorated. Therefore, the spray adheres to the wall surface of an inlet port, and enters a combustion chamber in the form of a liquid film, whereby there is a possibility to cause degradation of exhaust gas or degradation of controllability of engine output. [0006] In order to solve the above-described problems, in the device in the related art, it is an object of the present invention to provide a fuel injection valve for a gasoline engine in which atomization of fuel spray is improved while restraining variation in flow rate accuracy or spray characteristic, and maintaining the directivity of spray. Best Mode for Carrying Out the Invention [0007] A fuel injection valve according to the present invention including a valve element for opening and closing a valve seat and the valve element being driven upon reception of an operation signal from a control unit and causing fuel to be injected from a plurality of injection holes formed on an injection hole plate mounted on the downstream side of the valve seat, wherein the valve element is formed at a distal end thereof with a flat plane which is substantially parallel with the injection hole plate, wherein injection hole entrances of the injection holes are arranged inside an imaginary envelop along an intersection between an extensioji of a downstream inner wall of a seat portion of the valve seat and an upstream plane of the injection hole plate, and outside the flat plane at the distal end of the valve element, and wherein the relation between the vertical distance h between the flat plane at the distal end of the valve element and the upstream plane of the injection hole plate with the valve opened and the diameter d of the injection hole entrance is h[3 is established. Other configurations are the same as those in the first embodiment and description will be omitted. [0024] According to the third embodiment, the main flow 16a of fuel flow toward the injection hole plate 11 is guided toward, and caused to collide with, the outer periphery with respect to the injection hole entrances 12a, so that the flow 15a can be converted to a flow 16c along the injection plate, and the entrance angle y into the injection hole entrances 12a is increased. Therefore, separation of flow at the injection hole entrances is further enhanced, and the thickness of the liquid films is reduced, so that atomization of fuel spray is effectively accelerated. The third embodiment is also applicable to the fuel injection valve in the second embodiment as well as the first embodiment, as a matter of course. [0025] Fourth Embodiment Fig. 7 is a cross-sectional view of the fuel injection valve according to a fourth embodiment of the present invention. As shown in Fig. 7, in the fuel injection valve in the fourth embodiment, a plurality of dimples 19 are provided on the tapered plane 18 forming the valve included angle p with respect to the seat angle a so that the relation a>p is satisfied and being provided on the downstream side of the seat surface of the valve seat. Other configurations are the same as those in the first embodiment, and description is omitted. [0026] According to the fourth embodiment, since a small eddy current 20 is generated on the tapered plane 18, and the fuel flow passed through the seat surface 10c of the valve seat by the eddy current can hardly be separated on the tapered plane 18, the main flow 16a of the fuel flow can be further guided toward the taper surface. Consequently, collision with the injection hole plate 11 can be guided furtlier Lo the side of the outer periphery with respect to the injection hole entrances 12a, and the fuel flow which flows into the injection hole entrances 12a is converted into a flow which is parallel to the injection hole plate 11, whereby the entrance angle y to the injection hole entrances 12a may be further increased, so that atomization of fuel spray is enhanced. The fourth embodiment is also applicable to the fuel injection valve in the. second embodiment as well as the first embodiment, as a matter of course. [0027] Fifth Embodiment Fig. 8 is a cross-sectional view of the fuel injection valve according to a fifth embodiment of the present invention. As shown in Fig. 8, the fuel injection valve in the fifth embodiment is provided with a plurality of grooves 21 on the tapered plane 18 instead of the dimples in the above-described fourth embodiment, and the same effect as the fourth embodiment can be obtained. We claim: [1] A fuel injection valve comprising a valve element for opening and closing a valve seat, the valve element being driven upon reception of an operation signal from a control unit and causing fuel to be injected from a plurality of injection holes formed on an injection hole plate mounted on the downstream side of the valve seat, wherein the valve element is formed at a distal end thereof with a flat plane which is substantially parallel with the injection hole plate, wherein injection hole entrances of the injection holes are arranged inside an imaginary envelop along an intersection between an extension of a downstream inner wall of a seat portion of the valve seat, and an upstream plane of the injection hole plate and outside the flat plane at the distal end of the valve element/ and wherein the relation between the vertical distance h between the flat plane at the distal end of the valve element and the upstream plane of the injection hole plate with the valve opened and the diameter d of the injection hole entrance is h3 is established,and wherein a plurality of dimples or annular grooves are formed on the tapered plane having the valve included angle |3 on the downstream side of the seat surface of the, seat portion . [2] A fuel injection valve comprising a valve element for opening and closing a valve seat, the valve element being driven upon reception of an operation signal from a control unit and causing fuel to be injected from a plurality of injection holes formed on an injection hole plate mounted on the downstream side of the valve seat, wherein a projection which projects toward the downstream so as to extend substantially in parallel with the distal end of the valve element is formed at the center of the injection hole plate, and a flat portion is formed outside of the projection, wherein the relation between the minimum distance r from the distal end of the valve element to the center of the injection hole plate with the valve opened and the diameter d of the injection hole entrance is rp is established. [4] The fuel injection valve according to Claim 3, wherein a plurality of dimples are provided on the tapered plane having the valve included angle |3 on the downstream side of the seat surface of the seat portion. [5] The fuel injection valve according to Claim 3, wherein a plurality of annular grooves are formed on the tapered plane having the valve included angle p on the downstream side of the seat surface of the seat portion.