PURPOSE: To measure each channel characteristic of a multiplex filter even with one test output terminal by constituting the test circuit with an output selecting circuit and a switch group, connecting all other terminals of the switch group and using the connected terminal as an output terminal.

CONSTITUTION: Other terminals of switches S101∼S104 are connected in common and the common terminal is connected to the test terminal T0. Further, an output of an output selection circuit SE comprising input terminals T1, T2, inverters IN1∼IN6 and 3-input NOR gates No.1∼No.4 is connected to a control terminal of the switches S101∼S104. When the level of the input terminals T1, T2 is brought into a low level L respectively, a 1 waveform is outputted to the output of the 3-input NOR No.1 (No.2∼No.4 outputs are all at L level) and the swtich S101 of a switch group SWG is switched in synchronizing with the 1. When a signal is applied to the input IN, an output of a channel 1 in synchronizing with the clock 1 appears at the test terminal T0 so as to attain the characteristic test of the channel 1.