To stably cut a thin metallic sheet without the notches of cut surfaces and the adhesion of small spherical lumps by melts at the time of laser cutting.

The optical axis of a condenser unit 6 of a continuous oscillation laser beam 15 and the measurement direction of a distance measuring instrument 17 are arranged in parallel and the sheet 1 to be cut is held on X-Y tables 8, 8a perpendicular to the measuring direction of this distance measuring instrument 17. The X-Y tables 8, 8a are actuated and the perpendicular distances between the points at least ≥3 points on the sheet 1 to be cut and the distance measuring instrument 17 are measured. If all the perpendicular distances are within a permissible error range, the sheet 1 to be cut is cut as it is. If some of the perpendicular distances exceed this permissible error range, the posture of the sheet 1 to be cut is corrected on the X-Y tables 8, 8a until all of the perpendicular distances enter the permissible error range and, thereafter, the sheet 1 to be cut is cut by a continuous oscillation laser beam 15.