DESCRIPTION CLOTHES IRON STORAGE CASE TECHNICAL FIELD The present invention relates to an iron case that receives an iron. BACKGROUND ART In the related art, iron cases that receive an iron are generally equipped with a table where an iron is placed, a receiving case that surrounds the iron and has a lower opening closed by the table, and a locking mechanism that combines the table with the receiving case. An example of an iron case having a configuration that prevents the locking mechanism, which combines the table with the receiving case, from being unlocked when the case is lifted and carried (for example, see PTL 1), has been disclosed. According to the iron case described in PTL 1, a locking mechanism is unlocked by pressing an operating button, when a receiving case is separated from the table. In detail, the locking mechanism is unlocked by lifting up the receiving case, with the operating button pressed in the direction opposite to the direction in which the receiving case is lifted up and separated. Therefore, it has the advantage that the locking mechanism is not unexpectedly unlocked. However, it is difficult to unlock the locking mechanism when separating the receiving case from the table and it is inconvenient to use. An example of an iron case that makes it easy to unlock a locking mechanism in consideration of the problem is disclosed (for example, PTLs 2 and 3). A locking mechanism of an iron case of the related art which is described in PTL 2 is described with reference to Fig. 9. Fig. 9 is a cross-sectional view showing the main parts of a locking mechanism of an iron case of the related art. As shown in Fig. 9, receiving case51 includes operating button 52, locking body 55 having engagement groove 53 and claw 54 protruding upward at operating button52, and locking spring 56 biasing operating button 52 in the opposite direction to an unlocking direction (direction of an arrow AA). Table 57 has locking portion 58 having predetermined allowance (margin) in the vertical direction to be engaged with engagement groove53 of locking body 55. As receiving case51 is lifted up, locking portion 58 is fitted into engagement groove 53 of locking body 55 in response to relative separation of receiving case 51 and table 57 . Accordingly, locking body 55 is prevented from moving in the direction of arrow AA of Fig.9, which is the unlocking direction of locking body 55. Meanwhile, when receiving case 51 is separated from table 57, operating button 52 is operated in the direction of an arrow AA against the biasing force of locking spring56. Operating button 52 being in contact with the outer side of receiving case51 compresses locking spring 56. As a result, locking body 55 moves in the separation direction from table 57 and locking mechanism 59 is unlocked. Receiving case 51 is lifted up, with locking mechanism 59 unlocked by moving operating button 52 outward. Accordingly, receiving case 51 is separated from table 57. However, in the iron case having the configuration of the related art, operating button 52 is disposed on the outer side of receiving case 51 to be exposed. The upper end portion of operating button 52, which corresponds to a point P on the outer side of receiving case 51, is movably biased inward by locking spring 56, by being in contact with point P on the outer side of receiving case 51. Accordingly, when unexpected external shock is applied to receiving case 51 that is being carried, receiving case 51 is instantaneously elastically deformed. The position of operating button 52 biased by locking spring 56 is displaced (moved) and deviates from a predetermined position. As a result, the locking mechanism causes the following malfunction. That is, operating button 52 is deviated from a predetermined position by external shock applied to receiving case 51 during carrying. As a result, locking portion 58 and claw 54 that prevent locking body 55 from moving in the unlocking direction are damaged. Since operating button 52 is exposed on the outer side of receiving case 51, the degree of damage increases when external shock is applied directly. Hereafter, a mechanism that causes malfunction in locking mechanism 59 is described in detail. In general, receiving case 51 is made of thermally-resistant thermo-plastic resin, for example, ABS resin, which is strong against shock and has high elasticity in order to be able to receive a heavy iron that remains hot after being used. Therefore, the elastic deformation of receiving case 51 which is generated when external shock is applied is instantaneously restored. However, operating button 52 is disposed to be movable by locking spring 56 in receiving case 51. Therefore, when deviating from a predetermined position by external shock, operating button 52 remains in the deviated state. When a hot iron is received after being used, the internal temperature of receiving case 51 increases and receiving case 51 may also be heated, such that elastic deformation becomes easy to occur; therefore, the operating button is more easily moved from a predetermined position. These are factors that cause malfunction in locking mechanism 59. Therefore, the iron case having the configuration described above has a problem in that it is difficult to achieve both easy unlocking operability when separating a receiving case and at the same time preventing the malfunction in the locking mechanism with respect to the external shock. Citation List Patent Literature PTL 1: Japanese Patent Unexamined Publication No. 1-153189 PTL 2: Japanese Patent Unexamined Publication No. 7-124397 PTL 3: Japanese Patent Unexamined Publication No. 8-89699 SUMMARY OF THE INVENTION An iron case of the present invention includes a locking mechanism combining a table with a receiving case. The locking mechanism includes a locking portion formed at the table, a locking body engaged with the locking portion, an operating button operating the locking body, and a locking spring biasing the operating button to the table. Therefore, an iron case is achieved. According to this configuration, it is possible to make the operation for unlocking that lifts up the receiving case easy after moving the operating button outward. Further, it is possible to prevent malfunction of the locking mechanism by alleviate external shock applied to the locking body, by damping the external shock of the receiving case outside the locking body inserted in the locking portion. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a partial cross-sectional side view of an iron case according to a first embodiment of the present invention. Fig. 2 is a top view of the iron case. Fig. 3 is a bottom view of a locking mechanism of the iron case. Fig. 4 is a longitudinal cross-sectional view of the iron case, taken along line 4-4 of Fig. 3. Fig. 5 is an operational view of the iron case. Fig. 6 is a view of the iron case, seen from the arrow B of Fig. 5. Fig. 7 is an enlarged cross-sectional view of the locking mechanism of the iron case. Fig. 8 is a cross-sectional view taken along line 8-8 of FIG. 3, which shows a locking mechanism of an iron case according to a second embodiment of the present invention. Fig. 9 is a cross-sectional view showing the main parts of a lock mechanism of an iron case of the related art. EMBODIMENTS FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited by the embodiments. FIRST EXEMPLARY EMBODIMENT Fig. 1 is a partial cross-sectional side view of an iron case according to a first embodiment of the present invention. Fig. 2 is a plan view of the iron case. Fig. 3 is a bottom view of the locking mechanism. Fig. 4 is a cross-sectional view taken along the line 4-4 of Fig. 3. Fig. 5 is an operational view of the iron case. Fig. 6 is a view seen from arrow B of Fig. 5. Fig. 7 is an enlarged cross-sectional view of the locking mechanism of the iron case. As shown in Figs. 1 to 7, an iron case includes at least table 1, receiving case 3 with handle 4 at the upper portion, and locking mechanism 5 combining table 1 with receiving case 3. Iron 2 is placed on table 1, as shown in Fig. 1, with the front at an angle and underside 2a of iron 2 facing down. Receiving case 3 is made of thermally-resistant thermo-reversible resin, such as ABS resin, and receives iron 2 by surrounding and covering iron 2 placed on table 1 from the upper portion and closing opening 3a disposed in a lower portion by table 1. Handle 4 that is held by hand for carrying is disposed at the upper portion of receiving case 3 to be able to turn, as shown in Fig. 2. Locking mechanisms 5 are disposed in a pair to face, for example, the long sides of table 1 and receiving case 3, that is both sides in the front-rear direction of placed iron 2, close to opening 3a at the lower end of receiving case 3. Therefore, table 1 and receiving case 3 are separably combined. As shown in Figs. 3 and 4, locking mechanism 5 is composed of a pair of locking portions 6 disposed along both sides of the long sides of table 1, locking bodies 7 disposed at receiving case 3 and engaged with locking portions 6 from the lower portion, and operating buttons 8 disposed on both outer sides of the long sides of receiving case 3 to be movable outwardly. Locking portion 6 has locking piece 6a having a tongue shape, for example. Locking body 7 has protrusion 7a at the front end having a substantially rectangular shape to be engaged with locking portion 6 and is disposed at both sides in the front-rear direction of locking spring 9 disposed along a side of table 1. As shown in Fig. 4, locking body 7 is integrally formed with the lower portion of operating button 8 and at least projection 7a protrudes toward table 1 further than the inner side of receiving case 3 to be engaged with locking portion 6. Locking spring 9 disposed in receiving case 3 is bent, for example, in a substantially U-shape, with one end thread-fastened to operating button 8 by screw 9b and the other end biasing operating button 8 to table 1 in contact with spring hooking portion 13. As shown in Fig. 4, claw 8a disposed at the upper end portion of operating button 8 is inserted into hole 10, which is formed at a side of receiving case 3, from the outside. Claw 8a has first contact portion 11 being in contact with the outer surface around hole 10 and second contact portion 12 being in contact with the inner surface around hole 10 and is slidably engaged with the upper portion in hole 10 formed at receiving case 3. Hooking portion 9a that is cut downward and pulled outward is formed around the front end of locking spring 9. When locking spring 9 is inserted into spring hooking portion 13 formed in receiving case 3 from the lower portion, hooking portion 9a is fitted into receiving portion 14 formed in receiving case 3. In this state, when operating button 8 is moved down, the lower end of hooking portion 9a comes in contact with receiving portion 14, such that operating button 8 is prevented from separating from receiving case 3. Second contact portion 12 formed at claw 8a of operating button 8 keeps engaged with the inner surface of receiving case 3 by hole 10. Further, as shown in Fig. 7, length b defined by overlapping second contact portion 12 and the inner surface of receiving case 3 is set to be larger than distance a from the lower end of hooking portion 9a formed at locking spring 9 to receiving portion 14 of receiving case 3, such that the relationship of a