We have developed a low‐cost rapid‐scanning autocorrelator which enables us to monitor picosecond optical pulses up to almost 100 ps. To achieve wide‐scan range, each arm of a Michelson‐type interferometer is scanned 180° out of phase by using audio speakers, and also only one‐half of a full autocorrelation waveform of the pulses is observed. The present apparatus is usefully used to measure about 80‐ps pulses generated from a cw mode‐locked Nd:YAG laser.