A laser output command setting value from a control apparatus (17) is inputted to a current upper-limit value setting part (32) to set a current upper-limit value decided based on the laser output command setting value. A second comparator (30) compares a current command value, which is set from the laser output command setting value and from a measured laser output value measured by a power monitor (13), with the current upper-limit value set by the current upper-limit value setting part (32). It is arranged that when the second comparator (30) determines that the current upper-limit value is greater, the current command value be set as a reference current value; when the second comparator (30) determines that the current upper-limit value is smaller, the current upper-limit value be setas a reference current value; and that the reference current value is used to control a current to be supplied to an excitation means. In this arrangement, the current upper-limit value is set in accordance with the laser output command setting value, thereby preventing the current or energy to be supplied to the excitation means from increasing and hence suppressing the energy loss in any optical component to prevent the damage of the optical component from developing to such a degree that it must be displaced by a new component.