A roller pump comprises a pump housing having a cylindrical inner surface, a rotor fixed to a drive shaft placed at the central portion of the pump housing, rollers provided around the rotor and a driver for driving the drive shaft through a reduction gear. The rollers press an elastic tube installed between the rollers and the inner surface of the pump housing to transfer a liquid in the elastic tube by rotation of the rotor. The pump housing and the reduction gear are integrated into one body and an output shaft of the reduction gear is fixed to the drive shaft of the rotor. A stepping motor is used as the driver. A part of the inner surface of the pump housing is composed of a semicircle, another part of the inner surface is composed of a partial circle of which center is shifted from the center of the semicircle. The inlet and outlet slots are provided with levers for pressing the inlet or outlet portion into the inlet or outlet slot to hold inlet or outlet portion in the inlet or outlet slot respectively.