A differential pressure in a boundary portion between a streaked or linear fluid channel formed of a plurality of convex and concave portions disposed adjacent to one another in an undulated manner and a distribution channel for distributing a reactant gas or cooling water to be introduced into the plurality of fluid channels is reduced. In a structure of a separator of a fuel cell having a structure including streaked fluid channels formed of adjacent convex and concave portions formed on the surface of the separator, and a distribution channel which distributes, to these fluid channels, a fluid to be introduced into the fluid channels, in a boundary portion between the linear fluid channel and the distribution channel, a position of a terminal end of the convex portion constituting the fluid channel and a position of a terminal end of the concave portion are displaced in a streak direction of the fluid channel. It is preferable that the terminal end of the concave portion is positioned closer to the distribution channel than the terminal end of the convex portion. It is also preferable that the separator has a structure of such a type that front and back surfaces are integrated.