PURPOSE: To test a metal detector by moving a metal test piece along a guide tube which is extended through the section of an electromagnetic field and generating a distortion and a specific signal at the electromagnetic field.

CONSTITUTION: A yoke 2 where a single-wire coil is wound around the circumference generater an electromagnetic field when a nonmetal material or other products pass along a conveyer belt 1 and generates a distortion when a metal exists in the material, thus generating an alarm signal. Then, to test this device, a guide tube 3 for supporting, inside, a nonmetal slug 4 where a metal test piece 5 is buried is buried into the sealed body of the yoke 2 in parallel with the conveyer 1 and across the electromagnetic field. Then, the slug 4 is moved from one edge to the other at a certain time interval by an air pulse introduced via tubular junctions 7 and 8 provided at both edges of the tube 3, thus generating a response corresponding to the sensing means of the device. As a result, the device can be tested at this time interval.