To provide an expanding method in which a machined part is not broken in the vicinity of intersections of an expand lattice even when the height of the expand lattice is changed to a lower side.

A die 17 and a cutter 15 for expanding a long strip 2, and a feeder to feed the strip 2 in the longitudinal direction in an interlocking manner to the vertical motion of the cutter 15 are provided, and the strip 2 is expanded between the die 17 and the cutter 15. Expanding is performed while the length La of a non-expanded portion 2b between an expanded portion 2a and an adjacent expanded portion 2a of the strip 2 is set to be not more than the length Lb of a horizontal portion 16a at a leading end of a trapezoidal blade tip of a blade 16 of the cutter 15.