A physical quantity sensor has a semiconductor laser, a laser driver for providing, to the semiconductor laser, a driving electric current that is a waveform wherein the maximum portions and minimum portions of a triangle wave have been rounded; detecting means (a photodiode and a current-voltage converting amplifier) for detecting an electric signal that includes an interference waveform that is produced by the self-coupling effect between a laser beam that is emitted from the semiconductor laser and a return beam from a measurement object; and measuring means (a filter circuit, a counting device, and a calculating device) for calculating a physical quantity for the measurement object from interference waveform information.