In a rotary compressor of a vane rotary type, an outer peripheral surface of an eccentric portion of a shaft on a side adjacent to a center of the shaft is located radially inwardly of an outer peripheral surface of a main shaft inserted in a main bearing and that of an auxiliary shaft inserted in an auxiliary bearing. Also, a back clearance means used in mounting a piston on the shaft is provided in each of an inner peripheral surface of the piston and the eccentric portion of the shaft. Such configurations can reduce a diameter of the eccentric portion. A reduction in diameter of the eccentric portion can reduce a viscous force of oil acting between the eccentric portion of the shaft and the inner peripheral surface of the piston to thereby reduce a rotational moment about a center of the eccentric portion of the shaft, which rotational moment acts on the piston in a direction of rotation of the shaft, thus making it possible to reduce a sliding loss that is generated by a reciprocating motion of a vane in a vane groove.