Abstract: A direct-diode laser oscillator 1 can emit laser light to a laser processing head 3 based on the driving commands received from a system controller 5, which can control a laser robot 4 including the laser processing head 3. The direct-diode laser oscillator 1 includes the following: laser modules 10 each including a plurality of laser diodes connected in series or in parallel; a power supply circuit 20 for driving the laser modules 10 at a constant current, and a power controller 30 for controlling the power supply circuit 20 based on the driving commands and performing a fault diagnosis of the laser oscillator 1 based on which one of a plurality of previously divided regions of a diagnosis map the current and voltage applied to the laser modules 10 belong to.