To surely show whether metal parts, particularly, a block of wires exists or not by electrically insulating a plurality of spaced rods from structures for supporting them, connecting the rods to a power source and displaying flow of current.

In order to detect whether metal parts are positioned on a rod sieve 1, two spaced rods 1a, 2a are electrically insulated from structures 3, 4 for supporting the rods 1a, 2a by insulating materials 5, 6. The rods 1a, 2a are connected to a power source 7, on which a device 8 for displaying flow of current is provided. When the rods 1a, 2b are electrically conductively connected by a conductive metal parts positioned on the rod sieve 1, displaying is performed on the device 8. The device is not driven by relatively small metal parts. As a result, the device 8 drives in only a case where a block of wires exists such that function of subsequent dry distillation burning device parts may be obstructed.