A computation & control unit defines, as first and second voltages, A+B_signal voltages at distances which give the maximum and minimum voltages of an S_signal voltage, respectively. The unit further defines the higher one of the first and second voltages as an S_signal validity determination voltage and defines a distance at which the S_signal voltage takes on an in-focus determination voltage. The unit defines first and second lower limit voltages, defines the higher one of the first and second lower limit voltages as a lower limit voltage, and defines, as an upper limit voltage, an A+B_signal voltage at a distance which gives an in-focus determination voltage between the maximum and minimum positions of the S_signal voltage. An input and output unit outputs the upper and lower limit voltages as the upper and lower limit values of a settable range of the S_signal validity determination voltage.