PURPOSE: To accelerate conversion speed by connecting a gate consisting of only H-channel transistors to the parting point of a resistor string, and controlling the gate by outputting a value of more than source voltage from a tap decoder.

CONSTITUTION: A transfer gate is comprised of only the N-channel transistors M1-Mn. and the gates M1-Mn are selected by the tap decoder 5. The output of the decoder 5 is boosted to the value more than a source voltage by a booster circuit 6 to reduce the on-resistance of the gate. Potential selected by the decoder 5 is compared with an analog voltage inputted from an external terminal by a comparator 4, and a result is written on a register 2. Such comparison is continued by rewriting the value of the decoder 5 until a conversion result completion signal is sent out from a control circuit 1. Such a series of execution time is dependent on the on-resistance of the transistor and load capacity to the comparator 4. Thereby, fast conversion can be performed, and also, circuit configuration can be simplified.