A method of processing a substrate (1) made of a ferroelectric single crystalline material, including the steps of forming a desired proton-exchanged layer (2) in the substrate by proton-exchanging a portion of the substrate, and selectively removing the proton-exchanged layer to form a concave ditch structure (3) in the ferroelectric single crystalline substrate. The desired proton-exchange layer is formed using an acid containing a lithium salt as a proton-exchanging source and the surface of the substrate from which the concave ditch structure is formed is an X-cut surface or a Z-cut surface, as a main surface, of the ferroelectric single crystalline material used as the substrate. The concave ditch structure so formed can have its depth equal to or larger than its half opening width.