has an effective pressure sensing area s 1 to which suction pressure ps acts in the direction opposite to the arrow direction f 1 it is noted that any suitable means such as diaphragm may be used in place of the bellows 36 an the suction pressure ps sensitive means of the invention the rod 35 is slidable in the axial bore in the main valve portion 33 by contraction or expansion of the bellows 36 the main valve portion 33 has formed therein at an intermediate position thereof an axial bore 20 b into which the distal end of the rod 35 extends and first radial supply ports 20 a extending radially from the axial bore 20 b the first supply ports 20 a are in communication with the aforementioned first supply passage 20 formed through the cylinder block 1 and the rear housing 5 for communication with the crank chamber 9 the axial bore 20 b is formed with a cross sectional area s 2 the main valve portion 33 and the insert 37 define therebetween a discharge pressure chamber 52 as a discharge region which is in communication with the discharge chamber 4 through the second supply passage 23 formed in the rear housing 5 and second radial supply ports 23 a which are formed in the main valve portion 33 the first supply passage 20 the first supply port 20 a the second supply passage 23 and the second supply port 23 a.