To provide a connecting pin rotation stop mechanism wherein a connection pin is easily inserted/pulled out without using a finger and thus there is no danger of catching the finger during inserting/pulling out unlike the conventional case.

A guide plate 14 for detent is provided in the end surface of a connection pin 3 and the guide plate 14 is guided by a guide bracket 15 for attaching an expanded/contracted cylinder 8 to an upper frame so as to be slid only in the longitudinal direction. A pair of protruding pieces 14a are provided in the guide plate 14 and the protruding pieces 14a are guided by guiding slits 15a provided in the longitudinal direction of the guide bracket 15.