DESCRIPTION Title of Invention Plug pin receiving structure for power outlet and power outlet equipped with same structure Technical Field [0001] The present invention relates to a plug pin receiving structure for a power outlet and a power outlet equipped with the same structure. Background Art [0002] A multi-way power outlet and an adapter, which are equipped with several kinds of slots and plug pin receiving structures to be able to receive any one of the power plugs of different kinds of technical standards, are known in the prior art (for example, refer to Patent Literature 1). Such a plug pin receiving structure for the power outlet or the adapter comprises electrically connected and integrated conductors having a plurality of receiving portions. One of the receiving portions is selectively used at the time of use. A plug pin receiving structure of an adapter, to which two kinds of round pins can be inserted, is disclosed in the Patent Literature 1, where the structure is made by: setting two conductive strips in contact face-to-face mutually; fixing them at their center each other; and bulging their both sides into a shape of a character of 8 as a whole. Each of the bulged portions is used as a receiving portion. Moreover, two kinds of multi-way power outlets and their plug pin receiving structures are described with reference to FIGS. 21 to 23C, as other examples. [0003] FIGS. 21, 22A and 22B show a power outlet 93 which is compatible with both a plug 91 having three round pins 9 and a plug 92 having two round pins 9. The power outlet 93 comprises slots 9A, 9A and 9E for the plug 91 and slots 9B and 9B for the plug 92, i.e. five slots in total, and also two plug pin receiving structures and one plug pin receiving structure 95 behind these slots. Each of the plug pin receiving structures 94 and 95 has an aperture 9a and a lock-screw 9b which are used for inserting and connecting and also fixing a power source wire or an earthing wire. Moreover, the plug pin receiving structure 94 has receiving portions (A) and (B) for receiving the round pin 9 inserted from the slots 9A and 9B, respectively, and the plug pin receiving structure 95 has a receiving portion (E) for receiving the round pin 9 inserted from the slot 9E. These plug pin receiving structures 94 and are made of brass material by cutting work, and slits for expanding are prepared so that the receiving portion (A) etc. may be widened when the round pin 9 is inserted. [0004] FIGS. 23A, 23B and 23C show another power outlet 93 which is compatible with both two kinds of plugs (not shown) different in technical standards mutually and having three round pins. Each plug pin receiving structure 96 and 97 is arranged and used in a recessed space made in a holding member 93 a which is fixed to the rear face of the power outlet 93. Each of the plug pin receiving structures 96 and 97 has spaces made between two conductive plates placed to face mutually, and the spaces are used as receiving portions (A) and (B), and receiving portions (El) and (E2). For example, the plug pin receiving structure 96 comprises a plug pin receiving part 96a and a fixing part 96b. The receiving portions (El) and (E2) are formed in the plug pin receiving part 96a by press working of a conductive plate. The fixing part 96b has an aperture 9a and a lock-screw 9b used for inserting, connecting, and fixing a power source wire. The plug pin receiving part 96a is made by bridging a gap between two mutually facing conductive plates at lower portion thereof, and the plug pin receiving part 96a is fixed to the fixing part 96b by riveting the bridging portion and the top of the fixing part 96b together so that they become one unit. Citation List Patent Literature [0005] Patent Literature 1: JP2011 -524612A Summary of Invention Technical Problem [0006] However, there are following problems in the plug pin receiving structures in the Patent Literature 1 and FIGS. 21 to 23C mentioned above. The plug pin receiving structure shown in the Patent Literature 1 is a structure that the two receiving portions are deformed mutually and independently because the central fixing portion exists, and therefore the elasticity of the two conductive strips as a whole cannot be used effectively. The plug pin receiving structure shown in FIGS. 21 to 22B is made of brass material by cutting work, thus: its productivity is low; cost reduction is limited; it is difficult to have the durability for repeated use by suitably maintaining elasticity; and variations in product performance are likely to occur. The plug pin receiving structure shown in FIG 23 is widened at the time of insertion of the round pin 9 and holds the round pin 9 using the elasticity of the portion bridging the gap between the mutually facing two conductive plates and also the portion extended from there, and therefore the operation and the function thereof are greatly dependent on the elastic properties of the conductive plates. Thus, there is almost no degree of freedom in material selection, cost reduction is limited, and durability is deemed to be low in this plug pin receiving structure, too. [0007] The present invention is to solve the above problems, and an object of the present invention is to provide a plug pin receiving structure for a power outlet and a power outlet equipped with the same structure which can realize high productivity and reliability, and low cost by easy constitution. Solution to Problem [0008] In order to achieve the above purpose, the present invention provides a plug pin receiving structure for a power outlet, comprising: a pair of elastic plates made of electric conductor mutually placed in face to face position, wherein the elastic plates have joining portions for making the elastic plates into a ring shape by joining ends of the elastic plates, and a plurality of receiving portions each being arranged along between the joining portions and having a space into which a pin of a plug is to be inserted, and the elastic plates are deformed elastically when a pin is inserted into one of the receiving portions so that the space corresponding to the one expands, the space corresponding to the other gets smaller, and the inserted pin is pressed due to a restoring force of the elastic plates. [0009] The plug pin receiving structure for a power outlet preferably further comprises an expansion limiter for restricting expansion of a space between the elastic plates, which is arranged at an outer side of the elastic plate between the adjacent receiving portions. [0010] In the plug pin receiving structure for a power outlet, preferably the expansion limiter is a rib that is a part of a body for storing and holding the elastic plates, and a portion of the rib in contact with the elastic plate is circular and its radius of curvature is smaller than that of the elastic plate where the rib contacts. [0011] In the plug pin receiving structure for a power outlet, preferably the expansion limiter is a rib that is a part of a body for storing and holding the elastic plates, and a portion of the rib in contact with the elastic plate is circular and its radius of curvature is larger than that of the elastic plate where the rib contacts. [0012] In the plug pin receiving structure for a power outlet, preferably one of the joining portions has a bridge for connecting edges of the elastic plates, the edges are at a side opposite to a side of pin insertion, and the expansion limiter is arranged at the joining portion so that expansion of the space between the elastic plates is restricted at the side of pin insertion. [0013] Further, the present invention provides a power outlet comprising the plug pin receiving structure for a power outlet of the present invention. Advantageous Effects of Invention [0014] According to the plug pin receiving structure for a power outlet and the power outlet equipped with the same structure of the present invention, stable and highly reliable operations and durability can be realized by mutual co-working of each of parts of the elastic plates, because when a pin is inserted in either of the receiving portions arranged between the joining portions, each of parts of the elastic plates belong to the other receiving portions is elastically deformed as a whole at the same time. Since such a plug pin receiving structure can be made using an elastic plate thinner than a material in established practice, material cost can be suppressed, and also production can be made at low cost using a press working which generally exerts high productivity. Brief Description of Drawings [0015] FIGS. 1A and IB are perspective views showing situations that plugs of different technical standards are inserted, respectively, in a power outlet equipped with plug pin receiving structures for a power outlet according to an embodiment of the present invention. FIG 2 is a perspective view of the plug pin receiving structure for the power outlet. FIG. 3A is a plan view of the plug pin receiving structure, and FIG 3B is a plan view in a state where a round pin is inserted in the plug pin receiving structure. FIG 4 is a perspective view of a modification of the plug pin receiving structure. FIG 5A is a plan view of the modification, and FIG 5B is a plan view in a state where a round pin is inserted in the modification. FIG 6A is a plan view of another modification of the plug pin receiving structure, and FIG 6B is a plan view in a state where a round pin is inserted in the modification. FIG 7A is a plan view of another modification of the plug pin receiving structure, and FIG 7B is a plan view in a state where a round pin is inserted in the modification. FIG 8A is a plan view of another modification of the plug pin receiving structure, and FIG 8B is a plan view in a state where a round pin is inserted in the modification. FIG 9 is an exploded perspective view of a power outlet according to an embodiment of the present invention. FIG 10A is a front plan view of the power outlet, FIG 10B is a rear plan view of the power outlet, and FIG IOC is a side plan view of the power outlet installed on a wall. FIG 11A is a F-F line sectional view of FIG 10A, and FIG 11B is a G-G line sectional view of FIG 10A. FIG 12A is a perspective view of a plug pin receiving structure of the power outlet, and FIG 12B is a perspective view of a pillar terminal used for the plug pin receiving structure. FIG 13A is a perspective view of a plug pin receiving structure for a single receiving portion of the power outlet, and FIG 13B is a perspective view of a pillar terminal used for the plug pin receiving structure for the single receiving portion. FIG 14A is a perspective view of a plug pin receiving structure of the power outlet, and FIG 14B is a schematic plan view of the plug pin receiving structure in a state installed in a body of the power outlet. FIG 15 is a perspective view showing the body and the plug pin receiving structure of the power outlet in separate. FIG 16 is a perspective view in a state where the plug pin receiving structure is installed in the body of the power outlet. FIG 17 is a perspective view of the power outlet seen from a rear face. FIG 18 is an exploded perspective view of a modification of the power outlet. FIG 19 is a perspective view of a plug pin receiving structure of the modification. FIG 20 is a perspective view of the modification seen from a rear face. FIG 21 is a perspective view of a conventional power outlet. FIG 22A is a perspective view of the conventional power outlet seen from a rear face, and FIG 22B is a perspective view of a plug pin receiving structure of the conventional power outlet. FIG 23 A is a front view of another conventional power outlet, FIG 23B is a outline view of plug pin receiving structures installed in the conventional power outlet, and FIG 23C is a perspective view of one of the plug pin receiving structure. Description of Embodiments [0016] Hereafter, a plug pin receiving structure for a power outlet, and a power outlet equipped with the same structure according to an embodiment of the present invention are described with reference to drawings. FIGS. 1A, IB, 2 and 3 show the plug pin receiving structure 1 for a power outlet 10 and the power outlet 10 equipped with the same structure. As shown in FIGS. 1A and IB, the power outlet 10 is constituted by combining two of the plug pin receiving structure 1 for the power outlet 10. The power outlet 10 can accept two kinds of plugs 90A and 90B which are different mutually in a distance between pins 9 of the plug, and each of the plug 90A and the plug 90B is inserted exclusively in receiving portions (A) and receiving portions (B) of the plug pin receiving structures 1, respectively. However, both of two plugs 90A and 90B cannot be simultaneously inserted in the power outlet 10. [0017] As shown in FIG 2, the plug pin receiving structure 1 comprises two sheet of elastic plates 2 made of electric conductor placed face to face mutually, and this pair of the elastic plates 2 has two joining portions 21 for making them into a ring shape by joining the mutually facing ends of each elastic plate 2, and two receiving portions 22 ((A) and (B)) arranged along between the two joining portions 21, each of which has a space into which a pin 9 is to be inserted. The plug pin receiving structure 1 can hold a pin 9 in one receiving portion 22 using elastic forces of the elastic plates 2. The plug pin receiving structure 1 is a ring shape in a topological meaning, and may have a border space (X) between the mutually adjacent receiving portions 22, and the border space (X) may communicate mutually with the spaces of the receiving portions 22. Moreover, the plug pin receiving structure 1 is equipped with expansion limiters 3 for restricting expansion of a space between the elastic plates 2 but for the spaces at the receiving portions 22, which are arranged at each outer surface of the elastic plates 2 between the mutually adjacent receiving portions 22. That is, the expansion of the border space (X) between the elastic plates 2 is restricted by the expansion limiters 3. Moreover, a perimeter (peripheral length) of the ring of the plug pin receiving structure 1 made by joining the two sheets of the elastic plates 2 is set so that the pin 9 can be inserted only into any one of the two receiving portions 22. That is, the elastic plates 2 are made into a closed ring shape by the existence of the two joining portions 21, and thereby the perimeter of the area surrounded by the ring can be maintained at a predetermined constant value. Each of joining portions 21 is formed in a manner bonding surfaces of the ends of the elastic plates 2, for example. The expansion limiter 3 is set to contact with the elastic plate 2 at least at the time of insertion of the pin 9 to the plug pin receiving structure 1, and holds the elastic plate 2 slidably in a state the elastic plate 2 is in contact with the expansion limiter 3. In other words, the expansion limiter 3 functions as a guide for the deformation and movement of the elastic plate 2. [0018] The receiving portions 22 have a structure of a insertion guide of the pin 9 which works to widen the space between the elastic plates 2. That is, the pin 9 of a plug is, for example, what is called a round pin of cylindrical outer shape, and each receiving portion 22 has an opening of elliptic form, and the opening is equipped with chamfers of outward opening angles (not shown), etc. so that the pin 9 of a plug is guided for easy insertion. The plug pin receiving structure 1, seen from the direction of the pin 9 insertion, has a shape like a character number 8 but the center section is not joined (it is the same as a circle topologically), however, the elastic plates 2 may touch each other at the center section, i.e. at the border space (X). The elastic plates 2 have concave-and-convex structures smoothly continuing along the periphery between the two joining portions 21, and constitute the spaces of the receiving portions (A) and (B) and the border space (X) according to the concave-and-convex structures. The plug pin receiving structure 1 is manufactured by: making each elastic plate 2 of a plate material of electric conductor having a elasticity, for example, using a press-molding processing or a press working; setting the two elastic plates 2 facing each other; and joining end parts at both ends of them to make the joining portions 21 so that a ring shape is formed. Formation of the joining portions 21 at both ends can be performed using arbitrary bonding or connecting methods, such as by clinching, crimping, brazing, welding, bending, adhesion, riveting, and screw-clamping, for example. [0019] FIGS. 3A and 3B show operation of the plug pin receiving structure 1. As shown in FIG 3A, in a state a pin 9 is not inserted, the aperture of each receiving portion 22 is smaller than the size of the pin 9 (shown by a broken line in the figure). As shown in FIG 3B, for example, when the pin 9 is inserted in the receiving portion (A): the parts of the elastic plates 2 belong to the receiving portion (B) are deformed elastically, i.e. get smaller; thereby the insertion space for the pin 9 can be expanded; the pin 9 can go in against the forces from the elastic plates 2; and thus the pin 9 is held within the receiving portion (A) in a pressed state due to a restoring force of the elastic plates 2. In other words, by inserting the pin 9 into the receiving portion (A), as shown by arrows-a, the material composing each part of each of the elastic plates 2 belong to (in other words, around) the receiving portion (B) moves to a side of the receiving portion (A); the space of the receiving portion (B) gets smaller (narrower); the space of the receiving portion (A) expands (wider); and the pin 9 is stored within the receiving portion (A) in a pressurized state. Since each of the expansion limiters 3 restricts the expansion of the border space (X) and also the two joining portions 21 keep the perimeter of the ring made of the elastic plates 2, i.e. peripheral length, at a constant value, the pressure from the elastic plates 2 on the pin 9 inserted into the receiving portion (A) is increased by the effect of these restrictive conditions. [0020] According to the plug pin receiving structure 1, when a pin 9 is inserted in either of the receiving portions 22 formed between the joining portions 21, the elastic plates 2 corresponding to the other receiving portions 22 is deformed elastically in conjunction with the insertion. Consequently, by a mutual co-operative motion of each part of the each of elastic plates 2, the inserted pin 9 can be inserted at stable and high reliability. The co-operative motion is caused by the deformation of the whole of the elastic plates 2. The co-operative motion is a phenomenon that the material of the elastic plates 2 constituting one of the receiving portion 22 where a pin 9 is not inserted moves into the side of the other receiving portion 22 where a pin 9 is inserted, and this motion is equivalent to an operation under a substantially increased amount of material which presses the pin 9 at the receiving portion 22 pin inserted; in other words, the pin 9 is pressed with the pressure (elastic forces, restoring force) not only from the receiving portion 22 where the pin 9 is inserted, but also from the other receiving portion 22 where the pin 9 is not inserted. In the case that an elastic material of a small size is used, elastic fatigue will happen easily for the repeated use; decrease of contact pressure is fast; and therefore the reliability on durability will be inferior, because the pressing motion is accompanied with a rapid stretch of material in such a small size elastic material, in other words, with an elastic deformation of large stretch per material size. On the other hand, according to the plug-pin receiving structure 1, the reliability on durability can be improved without enlarging a material size, because the function of the elastic material can be enlarged substantially without enlarging the whole size. Thus, the co-operative motion in the plug pin receiving structure 1 can increase the reliability on durability of the plug pin receiving structure 1, even if a thinner plate is used for the elastic plate 2. Moreover, the thinner the elastic plate 2 becomes, the larger the mobility of the material of the elastic plate 2 becomes and the more effective the co-operative motion becomes. [0021] Moreover, since such a plug pin receiving structure 1 can be made with elastic plates 2 thinner than before, material cost can be brought down; the processing also becomes easier; and therefore low cost production is possible using a press working, etc. generally of high productivity. Since material mobility goes up as the elastic plate 2 becomes thinner, the plug pin receiving structure 1 can bring down cost and can improve the reliability on durability more effectively. Moreover, since the plug pin receiving structure 1 is equipped with the expansion limiters 3, according to backup effect thereof, a holding force (contact pressure to the pin 9) can be strengthened, or the material thickness of the elastic plate 2 can be made thinner without degrading the holding force. According to the plug pin receiving structure 1, its productivity can be made high, its quality can be stabilized, and its mold used can be made cheap, because the structure is easy. A power outlet can be constituted with space-saving by using the plug pin receiving structure 1, because the plug pin receiving structure 1 operates only within a plane orthogonal to the pin insertion direction, in other words, because the plug pin receiving structure 1 has spring feature or deforming feature only in directions (expanding or getting small directions of the receiving portions 22) within the plane and the material movement of the receiving portions 22 occurs in a limited area in the plane. [0022] FIGS. 4 and 5 show a modification of the plug pin receiving structure 1. As shown in FIG 4, the plug pin receiving structure 1 of this modification is not equipped with the expansion limiter 3 of the above-mentioned plug pin receiving structure 1 in FIG 2 etc. Although the expansion limiter 3 strengthens a holding force according to its backup effect, the expansion limiter 3 can be excluded when sufficient holding force is realized, for example, by increasing the intensity of the elastic plates 2. Such a plug pin receiving structure 1 operates as shown in FIGS. 5A and 5B, that is, the width of the border space (X) (distance between the elastic plates 2) increases from a width Wl at the time that the pin 9 is not inserted, to a width W2 at the time inserted (W1R2). Since according to the constitution of RR2 of FIG 8A, the structure of the elastic plate 2 has more nearly straight feature and the length LI in the longitudinal direction becomes shorter (L1R2 of FIG 8A, comparing with the constitution of R