PURPOSE: To attain direct D/A conversion of the serial digital signals by redistributing the sum of the charging electric charges of two capacities to these capacities after charging or discharging the 1st capacity up or down to a potential level corresponding to the logical level of an input digital signal.

CONSTITUTION: The 1st circuit C1 means consists of a constant voltage output circuit 2 and a switch element S1 which connects the output of the circuit 2 to the 1st capacity. The circuit 2 outputs the constant voltage Vin or 0 according to the logical level 1 or 0 of each bit of an input digital signal Din. This constant voltage is charged to the capacity C1 via the element S1. The 2nd circuit means consists of a switch S2 which connects the capacity C1 and the 2nd capacity C2 to each other. These capacities C1 and C2 are connected to each other when a switch element S2 is turned on. Then the sum of the charging electric charges of both capacities C1 and C2 is redistributed to these C1 and C2. Thus the direct D/A conversion is possible with the series digital signals.