A variable capacity type swash plate compressor has a shaft, a swash plate, pistons functionally connected with the swash plate, and a detector for detecting the reciprocating stroke of the piston. The reciprocating stroke of the piston is varied in accordance with an inclining angle of the swash plate in such a manner that a top dead point of the piston at a working chamber defined at one end of the piston is substantially constant. The detector detects the change of the width of the reciprocating stroke of the piston and calculating the capacity of the compressor. The detector employs a plurality of magnets provided on a center portion of the piston and an electromagnetic sensor provided on a cylinder block for facing to the magnets.