A laser machining method includes, before laser machining: calculating the amount of focus movement on the basis of a first measurement value measured with the external optical system warmed up and being the amount of energy of a laser beam passing through a small-diameter hole and a first reference value (database D1) predetermined depending on the type of contamination of the external optical system in relation to the first measurement value; and compensating the focus position in laser machining on the basis of the calculated amount of focus movement.