A period measurement system adapted to sample a biosignal at a predetermined sampling period, find an autocorrelation function for a variable .tau. from the sampled biosignal, and then find an autocorrelation function corresponding to the value of a phase difference variable obtained by changing the variable .tau. along a time axis. An autocorrelation function found in this manner is stored in memory and then compared with a subsequent-found autocorrelation function. The comparison operation is repeated for successive autocorrelation functions, thereby to find a peak of autocorrelation functions to measure the period of the biosignal.