Abstract: A gear motor includes a housing provided with a cylindrical opening therein. A cover covers the opening of the housing to form an enclosed chamber. A shaft is rotatably supported on the cover and housing. An outer tooth gear is axially movably mounted on the shaft and integrally rotatable with the shaft. An inner tooth gear meshes with the outer tooth gear to form actuating chambers. An inlet port is formed on the cover for introducing a high pressure fluid to the actuating chambers. An outlet port is formed on the cover for discharging the fluid from actuating chambers. High and low pressure ports are provided in both the cover and housing. The outer circumferential portion of the high pressure port of housing is larger in radius than that of the high pressure port of the cover.