A glass plate processing method for dividing a glass plate by a separation line that divides a main surface of the glass plate into two regions, includes: moving an irradiation point of a first laser beam along the separation line, and forming a crack extending from the separation line diagonally with respect to the main surface, in a cross-section orthogonal to the separation line; after the crack is formed, moving an irradiation point of a second laser beam along the separation line, and forming a modified portion, in the cross-section, on a virtual line extending in a direction perpendicular to the main surface, from a tip of the crack towards a center of a thickness of the glass plate; and after the modified portion is formed, applying stress to the glass plate and forming a new crack spanning from the tip of the crack to the modified portion.