PURPOSE: To realize smooth conveyance by providing a switch group and a control circuit for performing ON/OFF control of a power supply corresponding to a carrier sensor based on a detection signal from each carrier sensor thereby controlling a large number of primary coils through a small number of power supplies.

CONSTITUTION: Power supplies VA, VB are turned ON when the outputs of OR gates ORA, ORB have H level while turned OFF when the outputs have L level. When the power supplies VB, V13 comprise transistor inverters, base current conducts through the transistor if the output of connected OR gate has H level otherwise the base current is interrupted and thereby ON/OFF control is carried out. Primary coils A1, B1, A2, B2... are arranged along a conveyance path at substantially same pitch in the conveying direction of a carrier C. The primary coils A1, A2... are connected through switches SWA1, SWA2... with the power supply VA.