PURPOSE: To enable an easy observation of digital signals with various cycles mixed by synchronizing by starting the sweeping of a waveform observation equipment according to the output of a time counting circuit for counting cycles of an input electric signal.

CONSTITUTION: A time counting circuit 201 selects a time range t3 to be T1< t3<t2 between the cycle t1 of a smaller waveform and the cycle t2 of a larger waveform with a retriggerable monostable multivibrator. While an input signal 311 is repeated below the cycle t3, a time count output 312 of the multivibrator continue to be outputted and when the input signal 311 exceeds the cycle t3, the time count output moves to a high level after t3 from the last triggering point 301 and to a low level at the time 303 of the subsequent input signal. When the time count output 312 is supplied to a sweeping circuit 203 to start the sweep at the rising 302 or at the falling of 303, the part of the input signal 311 corresponding to the time 303 can be observed at the standstill with a display 204.