PURPOSE: To provide a doppler signal tuning method which achieves turning to a doppler signal rapidly when measuring a traveling speed, length, etc., of an object to be measured by the laser doppler system.

CONSTITUTION: A sensor 3 receives a scattering light b which is scattered from an object to be measured 1. Then, when taking out a doppler signal d which is included in an electrical signal A which is output from the sensor 3, the electrical signal A is passed through a band-pass filter 5 and is divided into an arbitrary number of frequency bands, only the electrical signal A above a threshold out of each electrical signal A which passed through the filter 5 is taken out, and then an electrical signal with the same frequency as the electrical signal A with the highest frequency out of the taken-out electrical signal A is taken out as the doppler signal d out of the electrical signal A which passed through the above band-pass filter 5.