PURPOSE: To quickly and surely detect a fault of an image conversion section having no correlation between input and output signals in the image converter having an image conversion section converting an image signal into a coded compressed signal or converting a coded compressed signal into an image signal.

CONSTITUTION: A fault monitor control means 2 selects an input signal changeover means 5 to receive a test signal in place of a substantial signal at first in the test mode and selects an output signal changeover means 6 to extract separately a processing signal from a coding processing path when the test signal is received. Then a fault monitor control means receives a test signal 7a from the input signal changeover means and collates a processing signal from the output signal changeover means with a collation signal 7b to discriminate the occurrence of a fault.