A laser apparatus comprising a laser diode module/cooling plate assembly in which a plurality of LD modules to which a driving current is supplied in series is disposed on the surface of a cooling plate, comprising: a laser power source; switch valves for switching a flow direction of a cooling liquid flowing through a cooling liquid flow path; a timing determination apparatus for determining the timing for switching the flow direction of the cooling liquid, by referencing temperature time series data of at least any one temperature variation part in the laser diode module/cooling plate assembly having temperature variations due to heat generated in a heat-generating part of the laser diode module; and a control circuit for outputting a driving current output command to the laser power source and outputting a valve switch command to the switch valves by referencing the determination result of the timing determination apparatus.