PURPOSE: To obtain perfect linearity within an operation range, by directly calculating the root of a specific equation showing the relation between the sampling value of an input signal and a pulse position.

CONSTITUTION: Sample holding circuits 1aW1c respectively hold an input signal having a pulse width T as a sample. A sample holding signal generating circuit 2 introduces a reference signal and respectively outputs the sample holding commands of the circuits 1aW1d at times t0, t0+T/2, t0+T, T0+3T/2 on the basis of said reference trigger signal. Next, an operator 3a introduces the respective outputs x1Wx3 of the circuits 1aW1c to output the value Z1 shown by formula I. An operator 3b introduces the respective outputs x2Wx4 of the circuits 1bW1d to output the value Z2 shown by formula II. A max. value detection circuit 4 introduces the outputs Z1, Z2 to output the larger output among both of them. By this method, perfect linearity is obtained within an operation range.