To reduce processing time required in a cutting process.

A first processing head 7A is moved along a first cutting line 101 to cut off a first side 201 of material glass 2, and a second processing head 7B is moved along a second cutting line 102 so as to be slightly delayed from the movement of the first processing head 7A to cut off a second side 202. That is, the adjacent sides 201 and 202 of the material glass 2 are cut off by the first and second processing heads 7A and 7B so as to slightly shift timing. In the same way, the third side 203 of the material glass 2 is cut off by the first processing head 7A, and a fourth side 204 is cut off by the second processing head 7B.