PURPOSE: To surely generate a digital signal regenerating an inputted analog signal and to generate an undesired digital data by providing a specific sampling pulse generating means on this A/D converter.

CONSTITUTION: A sampling pulse supplying device 18 consists of a delay circuit 20 delaying an original analog signal A1 by a prescribed time, a D/A converter 22 converting the digital signal D1 converted by an A/D converter 12 into an analog signal A11, a difference circuit 26 taking a difference between the output A12 of the delay circuit 20 and the output A11 of the D/A converter 22 and a variable frequency oscillation circuit 30. The oscillation circuit 30 converts the frequency of the sampling pulse D31 in response to a control signal A14 and its frequency is kept to an optimum value. Thus, the A/D conversion capable of reproducing the original wave form sufficiency with respect to the input analog signal including the high frequency component and when the changes in the input analog signal are reduced, the sampling number reduced and the useless production of digital data is prevented.