To provide a safety device for a slide frame capable of detecting the expansion/contraction state of the slide frame by one detection means and enhancing safety such as falling prevention of the vehicle by detection of the expansion/contraction state of the slide frame in a traveling vehicle having the slide frame expandable/contractable in a vehicle body widthwise direction.

A selector valve 25 is fixed to a cylinder part 22 of a width enlargement cylinder 20 for expanding/contracting a space between a pair of side frames not shown and a receiving member 26 for switching a switching position of the selector valve 25 is arranged on a piston rod 21 through a lengthy operation member 23. An operation piece 26a of the receiving member 26 is urged in a direction of the selector valve 25 by a spring 26b interposed between a flange part 28 and it. A slit is formed on the lengthy operation member 23 in a longitudinal direction and a guide piece 29 provided on the selector valve 25 is inserted into the same slit. By expansion of the width enlargement cylinder 20, the cylinder part 22 and the piston rod 21 are moved in a direction leaving from each other, the selector valve 25 and the receiving member 26 are abutted on each other and the maximum width enlargement state of the side frame can be detected.