Abstract: To improve a diode laser device comprising at least one laser diode, a power supply for the laser diode, a cooling device including a heat sink with the laser diode disposed thereon, and a coolant supply allowing a coolant to flow through the heat sink, so that simple monitoring of its operation is possible without interfering with use of the diode laser device, it is proposed that an operation monitoring device be provided, and that the operation monitoring device detect a quantity representing the electric current flowing through the laser diode and a quantity representing the temperature of the coolant exiting from the heat sink and determine from these an operational quantity representing operation of the diode laser device.