To provide an injection method for seed light in a coherent laser radar, in which a detecting capability in a heterodyne detection is increased or in which required output of a master laser can be reduced, by a method wherein the output of the master laser can be supplied to a slave laser as it is as the seed light and local light used for the heterodyne detection can be increased sufficiently.

A continuous output light 1a of a master laser 1 is introduced into an acousto-optic modulation element (AOM) 20. A prescribed voltage is applied to the AOM 20 only for a set time immediately before the transmission of pulse output light 3a of a slave laser 3. During the time, the continuous output light 1a is deflected and modulated so as to be injected into the slave laser 3 as seed light 1b. In other time, a voltage is not applied to the AOM 20. As a result, the continuous output light 1a which is passed through the AOM 20 is used for a heterodyne detection as local light 1c.