PURPOSE: To facilitate accurate adjustment without possibility of occurrence of an error even in the case of adjustment of a nonlinear signal processing circuit by using a stepwise test signal reaching a reference black level, a reference intermediate level and a reference white level within one horizontal period respectively.

CONSTITUTION: A timing control signal Stm is fed from a control circuit 20 to a test signal generating circuit 21 and a test signal Sst from the generating circuit 21 is fed to a t-side contact of a switch 11. The test signal Sst is at a reference black level for the initial part and the end part, a reference white level in the middle portion and a reference intermediate level at the intermediate portion before and after thereof within one horizontal period, and the test signal is repeated in the unit of 1H. The characteristic of a ν circuit 13, a gamma correction circuit 14 and a gain control amplifier 15 is adjusted based on the test signal by using an adjustment device 17 respectively. Thus, the possibility of occurrence of an error is precluded and the initial adjustment is easily and accurately implemented.