A method for dividing a brittle-material substrate, the method comprising: preparing a brittle-material substrate including a surface and having a thickness direction perpendicular to said surface, said surface being surrounded by a perimeter including first and second edges (ED1, ED2) opposite to each other; pressing a cutter edge (51, 51v) against said surface of said brittle-material substrate, said cutter edge including a protruding portion (PP, PPv) and a side portion (PS, PSv) extending from said protruding portion and having a convex shape, said pressing being performed so that, on said surface of said brittle-material substrate, said protruding portion of said cutter edge is positioned between said first edge and said side portion and said side portion of said cutter edge is positioned between said protruding portion and said second edge; forming a scribe line having a groove shape between a first position closer to said first edge of said first and second edges and a second position closer to said second edge of said first and second edges on said surface of said brittle-material substrate by causing said cutter edge pressed in said pressing to slide on said surface of said brittle-material substrate; after said forming a scribe line, forming a crack line; and dividing said brittle-material substrate along said crack line; wherein the method further comprises forming an assist line formed by a crack of said brittle-material substrate in said thickness direction, said assist line and said scribe line intersecting each other at said second position; characterized in that said forming a scribe line is performed by forming a scribe line apart from said perimeter, said crack line is formed by extending a crack of said brittle-material substrate in said thickness direction from said second position toward said first position along said scribe line, and wherein said forming a crack line is started by said assist line and said scribe line intersecting each other at said second position. preparing a brittle-material substrate including a surface and having a thickness direction perpendicular to said surface, said surface being surrounded by a perimeter including first and second edges (ED1, ED2) opposite to each other; pressing a cutter edge (51, 51v) against said surface of said brittle-material substrate, said cutter edge including a protruding portion (PP, PPv) and a side portion (PS, PSv) extending from said protruding portion and having a convex shape, said pressing being performed so that, on said surface of said brittle-material substrate, said protruding portion of said cutter edge is positioned between said first edge and said side portion and said side portion of said cutter edge is positioned between said protruding portion and said second edge; forming a scribe line having a groove shape between a first position closer to said first edge of said first and second edges and a second position closer to said second edge of said first and second edges on said surface of said brittle-material substrate by causing said cutter edge pressed in said pressing to slide on said surface of said brittle-material substrate; after said forming a scribe line, forming a crack line; and dividing said brittle-material substrate along said crack line; wherein the method further comprises forming an assist line formed by a crack of said brittle-material substrate in said thickness direction, said assist line and said scribe line intersecting each other at said second position; characterized in that said forming a scribe line is performed by forming a scribe line apart from said perimeter, said crack line is formed by extending a crack of said brittle-material substrate in said thickness direction from said second position toward said first position along said scribe line, and wherein said forming a crack line is started by said assist line and said scribe line intersecting each other at said second position.