A fuel cell separator comprising: a reactant gas supply manifold hole penetrating the separator in a thickness direction thereof; a reactant gas discharge manifold hole penetrating the separator in the thickness direction thereof; a groove-shaped first reactant gas channel provided on at least one main surface thereof such that the first reactant gas channel is bent, an upstream end of the first reactant gas channel is connected to the reactant gas supply manifold hole, and a downstream end of the first reactant gas channel is connected to the reactant gas discharge manifold hole; and one or more groove-shaped second reactant gas channels provided on at least one main surface thereof such that the second reactant gas channels are bent, at least upstream ends of the second reactant gas channels are connected to the reactant gas supply manifold hole, and the second reactant gas channels run along the first reactant gas channel; wherein the first reactant gas channel includes a first portion and a second portion located upstream of the first portion, the first portion is located closest to the upstream end, in a portion of the first reactant gas channel which lies between the downstream end and the second portion, and the second portion is located closest to the downstream end, in a portion of the first reactant gas channel which lies between the first portion and the upstream end; wherein the second reactant gas channel does not exist between the first portion and the upstream end, but exists between the downstream end and the second portion; and wherein a cross-sectional area of at least a continuous portion (hereinafter referred to as a first specified portion) of the first reactant gas channel, the continuous portion extending from the upstream end and/or a cross-sectional area of at least a portion (hereinafter referred to as a second specified portion) of the first reactant gas channel, the portion being located downstream of the first portion is/are smaller than a cross-sectional area of all portions of at least one second reactant gas channel (hereinafter referred to as specified second reactant gas channel) of the one or more second reactant gas channels. a reactant gas supply manifold hole penetrating the separator in a thickness direction thereof; a reactant gas discharge manifold hole penetrating the separator in the thickness direction thereof; a groove-shaped first reactant gas channel provided on at least one main surface thereof such that the first reactant gas channel is bent, an upstream end of the first reactant gas channel is connected to the reactant gas supply manifold hole, and a downstream end of the first reactant gas channel is connected to the reactant gas discharge manifold hole; and one or more groove-shaped second reactant gas channels provided on at least one main surface thereof such that the second reactant gas channels are bent, at least upstream ends of the second reactant gas channels are connected to the reactant gas supply manifold hole, and the second reactant gas channels run along the first reactant gas channel; wherein the first reactant gas channel includes a first portion and a second portion located upstream of the first portion, the first portion is located closest to the upstream end, in a portion of the first reactant gas channel which lies between the downstream end and the second portion, and the second portion is located closest to the downstream end, in a portion of the first reactant gas channel which lies between the first portion and the upstream end; wherein the second reactant gas channel does not exist between the first portion and the upstream end, but exists between the downstream end and the second portion; and wherein a cross-sectional area of at least a continuous portion (hereinafter referred to as a first specified portion) of the first reactant gas channel, the continuous portion extending from the upstream end and/or a cross-sectional area of at least a portion (hereinafter referred to as a second specified portion) of the first reactant gas channel, the portion being located downstream of the first portion is/are smaller than a cross-sectional area of all portions of at least one second reactant gas channel (hereinafter referred to as specified second reactant gas channel) of the one or more second reactant gas channels.