PURPOSE: To prevent various signal lines from mixture of noise components and to improve SN ratio by executing retrieving operation based upon the switching of a digit selecting signal only when the turn-on of a switch is detected.

CONSTITUTION: Only when the turn-on of any one of ten-key switches 1 arranged like a matrix is detected, logical "1" is supplied to the digit selecting signal lines S1WS3 cyclically and turned-on switch retrieving operation based upon the switching of the digit selecting signal is executed. In other cases, a fixed level logic "1" is supplied to all the digit selecting signal lines S1WS3 to make these lines steady state.