PURPOSE: To provide the input device for analog signal with which a comparator is stably operated even when the switching cycle of a switching signal generating circuit is made shorter in comparison with the time constant of a filtering circuit.

CONSTITUTION: This input device for analog signal is provided with a switch circuit to which plural analog signals Ain 1 and Ain 2 are connected, switching signal generating circuit 20 for applying a signal to select any one analog signal to this switch circuit, comparator 12 for comparing the signal selected by this switch circuit with an approximate value AV, register 15 for latching the output signal of this comparator at timing decided by a data updating clock, converting that signal to the digital signal of one bit and outputting it, D/A converter 13 for restoring the output signal of this register to an analog signal, and filtering circuit 14 for smoothing the analog signal converted by this D/A converter with the prescribed time constant and outputting an approximate value. This filtering circuit 14 is provided by one-to-one corresponding to the number of analog signals to be inputted, and a filter switching means 22 is provided to select any filtering circuit corresponding to the analog signal selected by the switching signal generating circuit and using it for smoothing.