PURPOSE: To improve reliability by duplicating a device by adopting a position detecting means whose system is different from that of a conventional position detector installed on the ground for the purpose of detecting the position of a traveling truck.

CONSTITUTION: At the time of detecting an rotational angle by combining a synchronizing transmitter 21 to a cable reel 5 mounted to the traveling truck 20, the rotational angle is in proportion to the traveling distance of the traveling truck 20. A position conversion means converts the rotational angle to a position and outputs a position signal with an old limit switch to duplicate the detection of the position so as to improve reliability. Furthermore, the position signal of the position conversion means and the position signal of the limit switch are collated to detect that one of the position detection means is in malfunction and that deviation is generated in the setting of both position detection means so that reliability is improved more.