PURPOSE: To prevent collision between hoists by means of impedances connected between trolley wire and detectors provided in each hoist.

CONSTITUTION: Trolley wire 2 divided into a plurality of sections is extended in parallel along power source wire, and an electric impedance is provided at each of the junctions between adjacent two sections of trolley wire 2, such that it can be detected from the impedance connection that hoists H1, H2 are present at adjacent two sections of trolley wire 2. However, output stignal is lowered between two sections spaced from each other by the presence of one further section of trolley wire 2, since there exist two impedances between them, so that presence of hoists H1, H2 in these spaced-apart sections can not be detected. Therefore, a rearward or succeeding hoist is permitted to pass through the section freely. With such an arrangement, it is enabled to operate a plurality of hoists safely without causing collision