Abstract: Disclosed is a small-sized pump device that is constructed of a motor constituting the small-sized pump device, a case member mounted on an output-shaft surface of the motor, a rotary circular-cylindrical member making a rotational movement due to the rotation of the motor, an air chamber formed of a soft elastic member, a drive member for compressing and expanding the air chamber, an intermediate case equipped with a suction opening and an exhaust opening, a suction valve and an exhaust valve for preventing the back flow of the air, and a case lid equipped with a suction opening and an exhaust opening. On the outer surface of the rotary circular-cylindrical member is provided a spiral guide portion engaged with a part of the drive member so that the drive member may be displaced at prescribed pitches in response to the rotation of the rotary circular-cylindrical member.