FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION (See section 10, rule 13) "FUEL CELL SEPARATOR AND FUEL CELL INCLUDING SAME" PANASONIC CORPORATION, a Japanese Corporation of 1006, Oaza Kadoma, Kadoma-shi, Osaka 571-8501, Japan The following specification particularly describes the invention and the manner in which it is to be performed. DESCRIPTION Technical Field [0001] The present invention relates to a fuel cell separator and a fuel cell including the fuel cell separator, and particularly to the configuration of the fuel cell separator. Background Art [0002] A polymer electrolyte fuel cell (hereinafter referred to as a "PEFC") causes an F electrochemical reaction between a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas, such as air, to generate electric power and heat at the same time. A unit cell (cell) of the PEFC includes a MEA (Membrane-Electrode Assembly), gaskets, and electrically conductive plate-shaped separators. The MEA is constituted by a polymer electrolyte membrane and a pair of gas diffusion electrodes (an anode and a cathode). [0003] Manifold holes (a reactant gas supply manifold hole and a reactant gas discharge manifold hole) are formed on a main surface of the separator. The manifold holes form manifolds through which the fuel gas or the oxidizing gas (each of these gases is called "reactant gas") is supplied and discharged. On a main surface contacting the gas diffusion electrode, a groove-shaped, serpentine reactant gas channel through which the reactant gas flows is formed so as to communicate with these manifold holes. [0004] A solid polymer electrolyte fuel cell separator is known, in which corner portions of turn groove portions of the serpentine reactant gas channel are omitted (see PTL 1, for example). Here, Fig. 8 is a front view schematically showing the schematic configuration of the solid polymer electrolyte fuel cell separator (hereinafter simply referred to as a "separator") disclosed in PTL 1. In Fig. 8, an upper-lower direction of the separator is shown as an upper-lower direction of the drawing. [0005] As shown in Fig. 8, in the separator disclosed in PTL 1, a distance from a boundary 333 between an independent flow groove portion 323 or 324 and a turn groove portion 321 up to an end 307 of a flow groove 320 decreases as the independent flow groove portion 323 or 324 separates from a boundary convex portion 325 formed between the independent flow groove portion 323 and the independent flow groove portion 324, the end 307 being located in a direction in which straight grooves of the independent flow groove portions 323 and 324 are extended. To be specific, convex portions 323a and 324a of the independent flow groove portions 323 and 324 extend so as to get close to the end 307 of the flow groove 320 as they separate from the boundary convex portion 325. The turn groove portion 321 is formed to have a substantially semicircular shape. [0006] By such a shape, in the separator disclosed in PTL 1, the gas having been supplied through the independent flow groove portions 323 to the turn groove portion 321 is supplied to the independent flow groove portions 324 through grid-like grooves of the turn groove portion 321. The gas and condensed water can be prevented from staying at a portion 3 50A located in the vicinity of an uppermost independent flow groove portion 323 A extending up to the vicinity of the end 307 of the flow groove 320. In addition, since the convex portions 323a and 324a of the independent flow groove portions 323 and 324 are extending, the contact area between the separator and the electrode is large, and the contact resistance can be reduced. Citation List Patent Literature [0007] PTL 1: Japanese Patent No. 4120072 Summary of Invention Technical Problem 10008] However, even in the separator disclosed in PTL 1, although the reactant gas having flowed through the uppermost independent flow groove portion 323 flows along the end 307, the reactant gas having flowed through the other independent flow groove portions 323 does not reach the end 307 and flows through the independent flow groove portions 324 located closer to the boundary convex portion 325. Therefore, the amount of reactant gas flowing from the upper portion 350A to a lower portion 350B in the vicinity of the end 307 becomes small. Especially, the reactant gas and the condensed water tend to stay at a hatched portion 350. [0009] Therefore, there is still room for improvement in the separator disclosed in PTL 1. [0010] The present invention was made to solve the above problems, and an object of the present invention is to provide a fuel cell separator capable of further preventing the reactant gas and the condensed water from staying at the turn portion and further reducing the electrical contact resistance between the separator and the electrode, and a fuel cell including the fuel cell separator. Solution to Problem [0011] In order to solve the above problems, a fuel cell separator according to the r present invention is formed to have a plate shape and includes a reactant gas flow region which is formed on at least one main surface of the fuel cell separator, includes a plurality of groove-like straight portions and one or more turn portions, and is formed to have a bent shape and through which a reactant gas flows, wherein: first rib portions are formed among the plurality of straight portions; the plurality of straight portion are formed such that the plurality of first rib portions are formed; the turn portion is formed so as to connect an upstream straight portion that is the straight portion located upstream of the turn portion and a downstream straight portion that is the straight portion located downstream of the turn portion and is formed such that the reactant gas flows from the upstream straight portion therethrough to the downstream straight portion; at least one of the one or more turn portions includes a gas mixing portion, a gas meeting portion through which the reactant gas from the upstream straight portion flows to the gas mixing portion, and a gas separating portion through which the reactant gas from the gas mixing portion flows to the downstream straight portion; the gas mixing portion includes a depressed portion and a plurality of protruding portions configured to protrude from a bottom surface of the depressed portion; second rib portions are formed in the gas meeting portion and the gas separating portion, base ends of the second rib portions being connected to the first rib portions, the second rib portions being configured to extend from the first rib portions; and when the first rib portion formed between the upstream straight portion and the downstream straight portion adjacent to each other is defined as a center rib portion, and a virtual line passing through a width-direction center of the center rib portion and extending in a direction in which the center rib portion extends is defined as a center line, the second rib portions are formed such that regarding two adjacent second rib portions, the length of the second rib portion located closer to the center rib portion is shorter than the length of the second rib portion located farther from the center rib portion in a direction in which the second rib portions extend, and an outermost second rib portion is formed so as to be bent inward toward the center line when viewed from a thickness direction of the fuel cell separator, the outermost second rib portion being the second rib portion formed in at least one of the gas meeting portion and the gas separating portion and being the second rib portion located farthest from the center rib portion. [0012] With this, the reactant gas and the condensed water can be prevented from staying at the outer peripheral portion of the turn portion. In addition, the electrical contact resistance between the fuel cell separator and the electrode can be reduced. [0013] In the fuel cell separator according to the present invention, in at least one of the gas meeting portion and the gas separating portion in each of which the outermost second rib portion is formed, a plurality of second rib portions may be formed so as to be bent inward toward the center line, and bent second rib portions that are the plurality of second rib portions formed so as to be bent may be adjacent to one another and be formed such that a distance between a tip end portion of the second rib portion located closer to the center rib portion and the center line is equal to or longer than a distance between a tip end portion of the second rib portion located farther from the center rib portion and the center line. [0014] In the fuel cell separator according to the present invention, the protruding portions may be provided so as to overlap one another when viewed from a direction in which the first rib portions extend. [0015] In the fuel cell separator according to the present invention, the protruding portions may be provided on extensions of the second rib portions. [0016] In the fuel cell separator according to the present invention, the protruding portions may be provided so as to overlap one another when viewed from a direction perpendicular to a direction in which the first rib portions extend. [0017] In the fuel cell separator according to the present invention, the protruding portions may be provided in a zigzag manner when viewed from a direction perpendicular to a direction in which the first rib portions extend. [0018] In the fuel cell separator according to the present invention, at least one or more protruding portions may be formed between a tip end of the outermost second rib portion and the center line. [0019] In the fuel cell separator according to the present invention, the outermost second rib portion may include a long portion extending from the first rib portion and a short portion extending from a tip end of the long portion inward toward the center line, and the short portion may be formed so as to extend along an outer end of the turn portion. [0020] In the fuel cell separator according to the present invention, the outermost second rib portion may be formed to have an L shape when viewed from the thickness direction of the fuel cell separator. [0021] In the fuel cell separator according to the present invention, the protruding portions may be provided on an extension of a groove formed between the short portion of the outermost second rib portion and the outer end of the turn portion. [0022] In the fuel cell separator according to the present invention, the protruding portions provided on the extension of the groove formed between the short portion of the outermost second rib portion and the outer end of the turn portion may be formed by a part of the outer end of the turn portion. [0023] In the fuel cell separator according to the present invention, each of the bent second rib portions may include a long portion extending from the first rib portion and a short portion extending from a tip end of the long portion inward toward the center line, and the short portion may be formed so as to extend along an outer end of the turn portion. [0024] In the fuel cell separator according to the present invention, the bent second rib portion may be formed to have an L shape when viewed from the thickness direction of the fuel cell separator. [0025] In the fuel cell separator according to the present invention, the reactant gas flow region may be formed such that the number of upstream straight portions is equal to or larger than the number of downstream straight portions. [0026] In the fuel cell separator according to the present invention, the second rib portions may be formed such that a distance between an outer end of the turn portion and an end of each of the second rib portions, the end being located closest to the outer end of the turn portion, increases as the second rib portion is located farther from the center rib portion in an arrangement of the straight portions. [0027] In the fuel cell separator according to the present invention, the reactant gas flow region may be formed to have a band shape as a whole. [0028] In the fuel cell separator according to the present invention, the reactant gas flow region may be formed to have a serpentine shape as a whole. [0029] In the fuel cell separator according to the present invention, in a case where S denotes an area of the gas mixing portion, A denotes a distance from a tip end of the second rib portion connected to the center rib portion up to an outer end of the turn portion, and B denotes a length of the turn portion in a direction perpendicular to the center line, the gas mixing portion may be formed such that the area S of the gas mixing portion satisfies S