PURPOSE: To test, with good efficiency, a semiconductor integrated circuit in which analog circuits and digital circuits are formed mixedly.

CONSTITUTION: An analog-signal output terminal at a D/A converter 4 and an analog-signal input terminal at an A/D converter 3 inside a semiconductor integrated circuit 1 in which analog circuits and digital circuits have been formed so as to be mixed are connected to an analog input/output short circuit switch 9. In a test operation, the analog input/output short circuit switch 9 is short-circuited under the control of a DSP 2; a series of reference digital data is given to a digital-signal input terminal at the D/A converter 4; a value which has fast-Fourier-transformed(F.F.T.) digital data which is output from a digital-signal output terminal at the A/D converter 3 is compared with an expected value; the A/D converter 3 and the D/A converter 4 are tested. The semiconductor integrated circuit in which the analog circuits and the digital circuits have been formed so as to be mixed can be tested easily and with good efficiency without connecting a measuring instrument at the outside.