A separator has a concavo-convex structure formed in mutually reversed shapes on two opposite sides thereof to define flow paths of different fluids on the respective two sides. The concavo-convex structure includes multiple first projections formed and protruded on one side of the two opposite sides and arranged at intervals having a preset regularity. The concavo-convex structure also includes multiple second projections formed and protruded on the other side of the two opposite sides in a specific area corresponding to an area for formation of the multiple first projections on the one side and arranged at intervals having a preset regularity. The concavo-convex structure further includes reinforcing elements protruded on the one side. Each of the reinforcing elements is formed as a convex in a specific shape of connecting multiple positions where the first projections are expected to be formed according to the preset regularity, while avoiding positions corresponding to the second projections formed on the other side. This arrangement effectively prevents a potential trouble caused by deformation of the separator due to a pressure difference between the flow pressures of the respective fluids flowing on the respective sides of the separator.