In this invention, a trench line (TL) is formed over a glass base plate (4) by rolling a scribing wheel (51) while applying a load (F). The scribing wheel (51) progresses towards a progression direction (DP), which is identical to the direction of an in-plane component (Fi) of the load (F). The step of forming the trench line (TL) is performed in such a manner as to obtain a crackless state. A crack of the glass base plate (4) in a thickness direction (DT) is caused to extend towards the progression direction along the trench line (TL), thereby forming a crack line (CL). In the glass base plate (4), in a direction that intersects with the trench line (TL), a continuous connection immediately below the trench line (TL) is disrupted by the crack line (CL). The glass base plate (4) becomes disjoined along the crack line (CL).
