output signal cout of the counter 1004 is applied to synchronizing signal frequency varying circuit 1020 and the output signal of said frequency varying circuit 1020 is applied to a synchronizing signal generating circuit cl the output of the counter 1004 is similar in wave form to the aforesaid one of fig7 when the output signal of the counter 1004 is a low level signal as shown in fig9 the frequency varying circuit 1020 produces a signal with a period of frequency f1 therefore the output signal sync is a signal of the frequency f1 when the counter 1004 has finished the counting of a prescribed time the output signal of the counter 1004 becomes a high level signal by this high level signal the frequency varying circuit 1020 produces a different frequency f2 from the aforesaid frequency f1 here assuming that f1 gt f2 the signal sync becomes an output wave form shown in fig9 therefore the frequency of the signal sync is f1 from the start of measurement of the distance to the prescribed time and thereafter becomes f2 so that the integration output of the integrator circuit becomes as shown by int in fig9 fig1 illustrates a concrete construction of the frequency varying circuit 1020 the frequency varying circuit 1020 includes an inverter 1021 a transistor 1022 resistors 1023 and 1024 and a capacitor 1025 the frequency varying circuit 1020 has the resistors 1023 and 1024 and the capacitor 1025 constituting a cr circuit and is arranged so that the input signal to the synchronizing signal generating circuit cl is altered.