[Technical Field of the Invcntion] [OOOl] The present invention relates to a metal plate with a screw hole, a housing, and a mcthod of manufacturing the metal plate with a screw hole. More particularly, the present invention relates to a metal plate with a screw hole that includes screw holes having excellent fastening force between the screw hole and a male screw and can prevcnt damage to the screw holes in spite of the reduction of the thickness of the metal plate, a housing including the metal plate with a screw hole, and a mcthod of manufacturing the metal plate with a screw hole. Priority is claimed on Japanese Patent Application No. 2013-154105, filed on July 25,2013, the content of which is incorporated herein by reference. [Related Art] [0002] Metal plates, which have been formed into various shapes by cutting or bending, are lrequently used in a housing of an outdoor unit of an air conditioner or a transformer. Since a screw structure is widely used at a mounting portion between this kind of metal plate and another metal plate of a housing or an attached component, therc is a case in which screw holes are formcd in the metal plate. 100031 FIG. 11 is a view schematically showing a conventional screw hole of a metal plate and is a longitudinal sectional view when viewed in a section including a central axis of the screw hole. In FIG. 11, a metal plate 5 10 including a screw hole is shown and a male screw 550 and another metal plate 551 including a through hole 55 la are shown by an imaginary line for easy understanding of the structure. The metal plate 5 10 shown in FIG. 11 includes a cylindrical vertical wall portion 5 11 serving as a screw hole (hereinafter, the vertical wall portion 51 1 may be referred to as a screw hole 5 11). One end of the vertical wall portion 5 11 is connected to a body 5 13 of the metal plate and a female screw 5 11 b is formed on the inner peripheral surface of the vertical wall portion 5 11. When the metal plate 55 1 including a through hole 55 la is mounted on the metal plate 5 10 so as to overlap the metal plate 510, the male screw 550 is inserted into the through hole 551a and screwed into thc screw hole 511 to join the metal plate 551 to the metal plate 510. Since the screw hole 51 1 can be , formed in the metal plate by burring and is most easily formed, the screw hole is widely used $ if FIGS. 12A and 12B arc longitudinal sectional views schematically showing j: '1I examples of steps of forming a conventional screw hole of a metal plate, of which FIG. 12A shows a drilling step, and FIG. 12B shows a burring step. When the screw hole 5 11 is to be formed, a prepared hole is formed first in a metal plate 520 that is a working object. The forming of the prepared hole can be performed by shcaring or cutting. When the prepared hole is formed by shearing, for example, a drilling punch 531 and a drilling die 532 shown in FIG. 12A are used. When the punch 531 and the die 532 approach each other, a part of the metal plate 520 is pushed into a through hole of the die 532 together with the punch 53 1. As a result, a part of the metal plate 520 is cut out and separated by blades of the punch 531 and thc die 532, so that the prepared hole is formed. [0005] A stretch flange, which forms a vertical wall portion, is formed at the metal plate 520, in which thc prepared hole has been formed, by burring. As shown in FIG. 12B, a burring punch 533, a burring die 534, and a blank holder 539 are used in burring. In thc burring, the punch 533, the die 534, and the blank holder 539 are moved so as to approach one another while the metal plate 520 is clamped by the die 534 and the blank holder 539. Accordingly, a peripheral portion of the prepared hole of the metal plate 520 is pushed into a recess of the die 534 together with the punch 533. For this reason, while being stretched in a push-in direction of the punch 533, the peripheral portion of the prcpared hole of the metal plate 520 is increased in a radial direction. As a result, the cylindrical stretch flange, that is, the vertical wall portion 5 11 is formed. [0006] It is possible to form the screw hole 511 in the metal plate 520 by forming the female screw 511b on the inner peripheral surfacc of the cylindrical vertical wall I I portion 511, which is formed as described above, using, for example, a tap (not shown). Conventionally, various screw holes have been proposed as a screw hole of a metal plate as described in, for example, Patent Document 1. Screw holes are formed in a second metal plate member among components of a housing proposed in Patent Document 1. The screw hole of the second metal plate member includes a first flat plate portion and a second flat plate portion that are formed so as to face each other by the folding-back of an end edge of the metal plate member. Tbc screw hole of the second metal plate member further includes: a first cylindrical portion that protrudes from the first flat plate portion in a cyliildrical shape and includes a screw-together portion on an inner peripheral surface thereof; and a second cylindrical portion that protrudes from the second flat plate poi-tion in a cylindrical shape, includes a screwtogether portion on an inner peripheral surface thereof, and is arranged substantially in series with the first cylindrical portion. According to the second mctal plate member, the fcmale screws are formed on the inner peripheral surfaces of the first and second cylindrical portions as the screw-together portions. Accordingly, it is described that tightening torque can be ensured by an increase of the axial dimension of a portion of the female screw coming into contact with a male screw. [OOOS] Further, Patent Document 2 discloses a structure in which, after a first thin plate including a first cylindrical portion and a second thin plate including a second cylindrical portion overlap each other so that the first cylindrical portion is coaxially fitted into the second cylindrical portion, a tapping screw is screwed together with the inner peripheral surface of the first cylindrical portion from the second thin plate and fastens the second thin plate to the first thin plate. [Prior Art Document] [Patent Document] [0009] [Patent Docun~cnt 11 Japanese Unexamined Patent Application, First Publication No. 201 1-214802 [Patent Document 21 Japanese Unexamined Patent Application, First Publication No. 2012-241 839 [Disclosure of the Invention] [Problems to be Solved by the Invention] [OOlO] In the conventional screw hole of thc metal plate 5 10 shown in FIG. 11, the female screw 5 11b is formed on the inner peripheral surface of the vertical wall portion (stretch flange) 5 11 formed by burring. A fastening force between the screw hole 51 1, which is formed by burring as described above, and a male screw is decreased with the reduction of the thickness of a mctal plate that is used Sor electric home appliances, automotive components, etc. and required to be reduced in weight. Further, there is also a case in which the screw threads of the screw hole 51 1 and the screw threads of the male screw 550 are not engaged with each other due to the deformation of the screw hole 5 11, that is, the screw hole 5 11 is damaged. [OOll] Furthermore, a screw hole, which includes the first and second flat plate portions disposed so as to face each other and includes the cylindrical portions formed on the first and second flat plate portions and including the screw-together portions, has been proposed as the screw hole in Patent Document 1. According to this structure, it is described that tightening torque can be ensured by an increase olthe axial dimension of a portion of the lemale screw coming into contact with a male screw. For the forming of this screw hole, a cylindrical portion needs to be formed by burring afier a prepared hole is formed in the first flat plate portion, a cylindrical portion needs to he formed by burring after a prepared hole is formed in the sccond flat plate portion, the first flat plate portion then needs to be folded back so as to overlap the second flat plate portion, and a female screw needs to be furthcr formed. Since worlting requires six steps as described above, the number of times of working is significantly increased. Moreover, since the ccnters of the two cylindrical portions need to correspond to each other when the first flat plate portion needs to be folded hack so as to overlap the second llat plate portion, high bending accuracy is required. For these reasons, there is a problem in that worlting costs are significantly increased when the screw hole disclosed in Patent Document 1 is formed in a metal plate. [0012] In addition, Patent Document 2 intcnds to prevent a relative position deviation between the first and second thin plates by the elimination of allowance between the first and second cylindrical portions as in "since the first cylindrical portion is swelled out in a radial direction and comes into pressure contact with the second cylindrical portion, the thin plates are firmly fixed to each other by screw members without looseness" described fn Paragraph No. [0014]. In this structure, when a dimensional difference between the inner diameter of the second cylindrical portion and the outer diameter of the first cylindrical portion is large, the first cylindrical portion cannot sufficiently come into pressure contact with the second cylindrical portion even though, for example, the first cylindrical portion is swelled out in the radial direction. As a result, there is a concern that the abovementioned effects may not be obtained. Accordingly, it is preferable to reduce the above-mentioned dimensional difference (clearance) to a certain level to obtain a sufficient fastening force. However, it is difficult to fit the first cylindrical portion into the second cylindrical portion in this case, and in some cases, there is also a concern that a tapping screw may be screwed by mistake even though the firs1 cylindrical portion is not appropriately fitted into the second cylindrical portion. In this case, naturally, the first and second thin plates cannot be reliably fixed to each other. Moreover, since this kind of screw hole is generally provided not only at one position but also at a plurality of positions, it is not possible to deny that the probability of this operational error is not low. Accordingly, the structure of Patent Document 2 is also not necessarily sufficient in terms of reliably obtaining a sufficient fastening force. [0013] The invention has been made in consideration of the above-mentioned circumstances, and an objcct of the invention is to provide a metal plate with a screw hole which can hold back an increase in working costs and of which the screw holes arc excellent in a fastening force between the screw hole and a male screw and in which damage to the screw holes can be prevented in spite ofthe reduction ofthe thicltness of the metal plate, a housing including the metal plate with a screw hole, and a method of manufacturing the metal plate with a screw hole. [Means for Solving the l'roblem] [0014] Tn order to solve the above-mentioned problem, the inventors obtained the following factors (a) to (d) by analyzing the phenomena of the decrease in a fastening force of thc screw and damage to the screw that are caused by Lhe reduction of the thicltness of a conventional metal plate described with reference to FIG. 11. (a) In burring, the thickness of the root portion of the vertical wall portion 51 1 is decreased as the cylindrical vertical wall portion 51 1 is formed. Meanwhile, "the root portion" means a portion 5 11 a, which is connected to the body 5 13 of the metal plate, of the vertical wall portion 51 1 as surrounded by a two-dot chain line in FIG. 11. (h) A screw groove is formed to the root portion of which the thickness is decreased due to the decrease in the thickness of (a), so that a very thin portion is formed. (c) The very thin portion is elastically dcformed, so that a fastening force is decreased. Further, when the male screw 550 is screwed (tightened), there is a case in which the very thin portion is plastically dcformed and the screw hole 51 1 is damaged or broken. (d) When the male screw 550 is screwed, loosened, or the like, there is a case in which the cylindrical vertical wall portion 511 is plastically deformed in a diameterincreasing direction and a contact area between the screw threads of the screw hole 51 1 and the screw threads of the male screw 550 is decreased. In this case, there is also a case in which a fastening force is decreased and the screw hole 5 11 is damaged [00 151 As a result of earnest investigation based on these phenomena, the inventors have conceived that a screw hole is formed as a double wall including a cylindrical outer wall portion rising from a metal plate and a cylindrical inner wall portion folded back inward from the outer wall portion and a female screw is formed on the inner peripheral surface of the inner wall portion. Accordingly, even when the thickness of the metal plate is small, the screw hole is excellent in a fastening force between the screw hole and a male screw and damage to the screw hole can be prevented. Further, the inventors have conceived that substantially the same effects are obtained even when another aspect ofthe screw hole is formed as a double wall including a cylindrical inner wall portion rising from a metal plate and a cylindrical outer wall portion folded back outward from the inner wall portion and a female screw is formed on the inner per~pherals urface of the inner wall portion. [0016] The invention has been completely made on the basis of the above-mentioned knowledge, and the gist thereof will be described below. [0017] (1) According to one aspect of the invention, there is provided a metal plate with a screw holc. The screw hole includes a cylindrical outer wall portion that rises from one surface of the metal plate and a cylindrical inner wall portion that is folded back to the inside of the outer wall portion from an end cdgc of the outer wall portion, and a female screw is formed on an inner peripheral surface of the inner wall portion. (2) In the metal plate with a screw hole according to (I), an end of the inner wall portion and the other surface of the metal plate may be made flush with each other. (3) In the metal plate with a scrcw hole according to (I), an outer peripheral surface of the inner wall portion may be separated from an inner peripheral surface of the outer wall portion when a male screw is not screwed together with the female screw of the screw hole, and at least a part of the outer peripheral surface ol the inner wall portion may come into contact with the inner peripheral surface of the outer wall portion when the male screw is screwed together with the female screw of the screw I 1 I hole. ! (4) In the case of the metal plate with a screw hole according to (3), an end of the inner wall portion and the other surface of the metal plate may be made flush with each other. [0018] (5) According to another aspect of the invention, there is provided a metal plate with a screw hole. The screw hole includes a cylindrical inner wall portion that rises from one surface oS the metal plate and a cylindrical outer wall portion that is folded back to the outside of the inner wall poition from an end edge of the inner wall poition, and a female screw is formed on an inner peripheral surface of the inner wall portion. [00 191 i (6) According to still another aspect of the invention, there is provided a housing including: a metal plate with a scrcw hole; a member that includes a through hole; and a male screw that is screwed into the screw hole whilc being inserted into the through hole and joins the metal plate to the member including the through hole. The metal plate is the metal plate with a screw hole acco~dingto any one of (1) to (5). [0020] (7) According to still another aspect ofthe invention, there is provided a method of manufacturing the metal plate with a screw hole according to any one of (1) to (4). The method includes: a step of forming the outer wall portion and a disc portion which is connected to an inner portion of the outer wall portion, at the metal plate; a step of forming a prepared hole at a center of the disc portion; a step of forming the inner wall portion by bending the disc portion, in which the prepared hole has been formed, to the inside of the outer wall portion; and a step of forming the female screw on the inner peripheral surface of the formed inner wall portion. (8) In the method of manufacturing the metal plate with a screw hole according to (7), a punch, which has a dimension of a shoulder having a radius R (mm) satisfying the following expression (a), may be used in the step of forming the outer wall portion and the disc portion at the metal plate having a thickness t (mm). 0.5ilUticen of Tables 1 to 3 are as follows: VG (Very Good) means that "TBlTA21.2" is satisfied. GD (Good) means that "1.2>TBITA~.IO n is satisfied. FA (Fair) means that "l.O>TBITA>O.X" is satisfied. BD (Bad) means that "TB/TA