PURPOSE: To shorten the diagnosing time of an image memory by using an image display address as a memory address at the time of diagnosis.

CONSTITUTION: At the time of diagnosis, diagnosing data is set at an image memory 4 and a diagnosing circuit 10 as memory write data. Then a picture display address is converted into a memory address and a memory control signal is converted into a write timing signal by a memory control signal converting circuit 8. The memory 4 writes the diagnosing data as the memory write data by a memory address converted from the picture display address and the write timing signal. Then the memory control signal is converted into a read timing signal and the memory 4 reads the memory read data by the read timing signal and the memory address. The circuit 10 compares the memory read data with the diagnosing data and decides the occurrence of some errors if no coincidence is obtained between both data and then outputs a deciding signal.