The velocity calculating device includes a semiconductor laser for emitting a laser beam at a web that is the subject to be measured; a photodiode for converting into an electric signal the optical power of the semiconductor laser; a lens for focusing and emitting the beam from the semiconductor laser and for focusing the return light from the web and injecting it into the semiconductor laser; a laser driver for driving the semiconductor laser; a current-voltage converting/amplifying portion for converting the output current from the photodiode into a voltage and then amplifying; a filter portion for removing the carrier wave from the output voltage of the current-voltage converting/amplifying portion; a signal extracting portion for counting the number of interference waveforms included in the output voltage of the filter portion; and a calculating portion for calculating the velocity of the web based on the counting result of the signal extracting portion.