A laser irradiation method sets scan lines in an x direction in parallel, and in a y direction to be separate by an inter-scan-line distance Py corresponding to laser irradiation areas of a processing target object, orients a length direction of a linear laser spot with length Wy and width Wx in the y direction, and irradiates target object with the laser spot in each of irradiation positions arranged at width direction intervals Λ while moving the laser spot relative to the target object along the scan lines. The method includes determining the inter-scan-line distance Py, the width direction interval Λ, and a position shift quantity Δx (where, 0