A laser apparatus (100) for performing the laser machining operation by condensing the laser light output from a laser oscillator (3) is disclosed. A laser output value calculation unit calculates a laser output value (L1) based on a command value (L0) issued to a laser oscillator (2). A temperature change estimating unit (31) estimates the temperature change or the temperature (Te) of specified component element(s) (7a, 7b) of the laser apparatus based on the elapsed time (t) and the laser output value calculated by the laser output value calculation unit. An adjusting unit (32) adjusts the conditions for controlling the laser or the conditions for laser machining based on the temperature change or the temperature of the specified component element estimated by the temperature change estimating unit. A stable laser machining operation is performed without a temperature sensor. The laser output value (L1) may be measured by a laser power sensor (5).