PURPOSE: To absorb and relax pressure pulsation of a hydraulic pump for driving an auxiliary machine without separately mounting an accumulator.

CONSTITUTION: A pulsation absorbing chamber 3 is recessed in the side of a cylinder block 1 and a mounting seat surface 2 is formed on the periphery of the pulsation absorbing chamber. A hydraulic pump 4 is mounted in a manner to cover the pulsation absorbing chamber 3 therewith. The pulsation absorbing chamber 3 is widely communicated to the delivery port 8 of the hydraulic pump 4. An oil pressure passage 9 running from the bottom of the pulsation absorbing chamber 3 to a hydraulic motor 11 for driving an auxiliary machine is formed in the cylinder block 1.