PURPOSE: To obtain an initial activating device to lightly move a beam carrier at beginning of pushing by driving a wheel slightly rotatingly with pressure fluid.

CONSTITUTION: At beginning to push a beam carrier, pressure fluid is introduced in a cylinder 3 to push a drive shaft 4 threadedly engaged with a piston 32, the engaging/disengaging shaft part 42 of the drive shaft 4 is fitted into a one- way clutch 8, advancing of the shaft part 42 is stopped by abutting to a stopper 29, screw thrust is generated between the internal thread 35 of the piston 32 and the threaded shaft part 41 of the drive shaft 4, and rotation of the drive shaft is transmitted to a wheel 2 through the one-way clutch 8 and an axle 24. Although the wheel 2 is forcedly rotated by a little stroke which the piston 32 slides by itself, after once moving the beam carrier can be lightly moved only through pushing by an operator.