PURPOSE: To detect effectively presence/absence of a form even if a carrying path has a high temperature, by providing a rugged part on the surface of a corona current accepting electrode which faces a corotron and forming an insulating layer due to air between the carried form and the electrode.

CONSTITUTION: A corotron 10 and a corona current accepting electrode 20 are so arranged that they face each other with the carrying path of a form PA between themselves, and feeding is performed properly to generate a certain electric field between them, and presence/absence of the form PA is detected on a basis of variation of the quantity of current flowed to the electrode 20. Proper insulator blocks 22 are arranged interruptedly on an electrode part 21 of the electrode 20 to form the rugged part. Thus, the insulating layer due to air is formed between the form PA and the electrode part 21 even if the form PA exists on blocks 22 for a long period.