19 the backpressure introducing valve 28 is a check valve which allows an oil flow from the radially extending oil passage 29 side to the compression side backpressure chamber 25 side the backpressure introducing valve 28 includes an orifice 28 a through which the radially extending oil passage 29 and the compression side backpressure chamber 25 are in constant communication with each other further a radially extending oil passage 30 is provided through an side wall of the piston bolt 4 the radially extending oil passage 30 is connected to the compression side oil passage 9 through a compression side orifice 31 which is provided at the piston 3 an extension side valve body 32 extension side pressure control valve first valve body facing the radially extending oil passages 18 and 20 and a compression side valve body 33 compression side pressure control valve second valve body facing the radially extending oil passages 29 and 30 are slidably fitted in the guide bore 19 of the piston bolt 4 a valve seat member 34 is screwed in the distal end of the piston bolt 4 a solenoid actuator 35 damping force adjuster is disposed inside the case 6 a front end of the extension side valve body 32 abuts against.