A gas separator having excellent humidifying performance, pressure loss, volume efficiency, and durability humidifies gases used in an on-vehicle fuel cell. The gas separator has upper and lower surfaces of pleat elements of a pleat molding covered by a plate having intake and exhaust ports. The pleat molding is formed by pleating a compound membrane formed of a gas separating membrane and at least one layer of permeable reinforcement material. The pleat element is formed by disposing a reinforcement frame at the outer side part of the molding. The ratio (R=L/H) of a shortest distance (L) between the intake and exhaust ports to a height (H) of the pleat molding is 0.1 to 7.0, and the ratio (W/Le) of the width (W) to the length (Le) of the pleat molding is 0.3 to 10.0. The gas separator operates at a volume flow ratio of 200 or higher.