A laser machining apparatus in which laser output can be easily corrected in view of secular change of components of the laser machining apparatus and influence of temperature. Output values of a laser oscillator operated according to a reference laser output command value Psc are stored as reference output values P(.theta.) at different temperatures .theta. in a table TB1. When the laser oscillator is started up, a measured output value Pr of the laser oscillator supplied with the reference command value Psc is obtained, and a new correction coefficient k is obtained to replace an old one. From a reference output value P(.theta.) at a temperature at that time and the actual output measured value Pr, a correction value .DELTA.P is obtained and stored. Each time temperature .theta. is measured, a reference output value (P.theta.) at a measured temperature .theta. is obtained from the table TB1, an estimated output value Pa is obtained by subtracting the correction value .DELTA.P from the reference output value (P.theta.), and a new correction coefficient k is obtained to replace an old one. In other words, an output value at a different temperature is estimated on the basis of the correction value .DELTA.P obtained when the laser oscillator is started up, and a new correction coefficient k is obtained to replace an old one. When the temperature reaches a predetermined temperature, a new final correction coefficient k is obtained on the basis of the reference command value and a measured output value to replace an old one.