PURPOSE: To provide a means preventing a skew in a straight advance action caused by the drive of a working device at a low cost.

CONSTITUTION: In the hydraulic circuit of a back hoe constituted with a group of travel and working device control valves on center-bypassed multivalve type valve blocks 3, 4 and feeding the pressure oil to a group of the control valves in parallel, check valves 16 blocking the inflow of the pressure oil to the travel control valve side are provided on pressure oil feed passages 17, 18 respectively at portions between the branch feed passages 9a, 13a of the travel control valve and the branch feed passages 10a, 14a for the working device control valve downstream of the pressure oil feed to bypass passages 3A, 4A. A selector valve 5 capable of connecting or disconnecting the pressure oil feed downstream side portions of the check valves 16 is provided, and a switching mechanism 19 switching the selector valve 5 excited to the closed position to the opened state in conjunction with a straight advance action is provided.