Dual capacity compressor including a power generating part (20) having a reversible motor (21, 22) and a crank shaft (23) inserted in the motor (21, 23), a compression part (30) having a cylinder (32), a piston (31), and a connecting rod (33), a crank pin (110) in an upper part of the crank shaft (23) eccentric to an axis of the crank shaft, an eccentric sleeve (120) rotatably fitted to the crank pin (110), and the connecting rod (33) respectively, a key member (130) for positive fastening of the eccentric sleeve (120) to the crank pin (110) in all rotation directions of the motor, and an eccentric mass (200) provided to the eccentric sleeve (120) for rotating the eccentric sleeve (120) around the crank pin (110), thereby preventing relative motion between components that sustain the eccentricity, and permitting a smooth motion between the components.