To provide a vehicular air conditioning compressor for preventing the looseness of a center bolt relative to a shaft during the operation of torque in opposition to the driving direction of a compressor.

The vehicular air conditioning compressor includes a pulley (3C) to which part of the power of an engine (1) is transmitted via a pulley belt (2), a clutch disc (5) on which the pulley (3C) abuts when energized, and from which it is isolated when deenergized, the shaft (4) mounted on the clutch disc (5), and the center bolt (7) fastened to the shaft (4) to clamp the clutch disc (5), wherein a looseness preventing mechanism (M1) is provided for preventing the looseness of the center bolt (7) during the operation of toque in opposition to the driving direction of the compressor (101).