PURPOSE: To eliminate the necessity of a high-resolution AD converter, by obtaining high-resolution AD-converted data by using an existing low-resolution AD converter.

CONSTITUTION: Under the control of an AD-conversion control section 9, a multiplexer (MPX) 1 leads analog signals to a sample holding circuit 2. An n-bit AD converter 4 outputs AD-converted data 14a upon receiving the output of the circuit 2 and latches n-bit digital data in an n-bit register 5a. The digital data are DA-converted by an n-bit DS converter circuit 6 and outputted to a differential amplifier 7. The amplifier 7 amplifies the difference voltage between the outputs of the circuits 6 and 2 by 2n times and outputs the amplified difference voltage to a sample holding circuit 8. Accordingly, the AD-converted data of low-order bits which are AD converted by the converter 4 are obtained. The output of the amplifier 8 inputted to the circuit 8 is sample- held and inputted to the MPX 1 as analog signals 18. Then the MPX 1 outputs the signal 18 to the circuit 2. Hereafter, the AD conversion is performed by means of the converter 4 similarly and the data of the 2nd time are latched in an n-bit register 5b by an AD-converted data latching signal 19f. Therefore, an output which is obtained by combining the n-bit outputs of the registers 5a and 5b becomes a 2n-bit AD- converted output 15.