To provide a compact detaching/attaching device of a working machine capable of easily performing the extracting/inserting operation of a connection pin and having a connection part not causing oil leakage.

A pair of right and left fixed pistons (5, 5) and a pair of right and left cylinder pins (7, 7) fitted to the pair of right and left fixed pistons (5, 5) and laterally slidable in a pair of right and left bosses (1, 1) are installed in the pair of right and left bosses (1, 1) at connection ends of two working machines (10, 20) fixed to one working machine (10). The pair of right and left cylinder pins (7, 7) are inserted into and extracted from the pair of right and left bosses (21, 21) fixed to right and left sides of the other working machine (20) at the connection ends by a hydraulic force.