PURPOSE: To provide a method for forming an opening in glass in which an opening of sashed flat glass can safely and rapidly be formed.

CONSTITUTION: Flat glass 4 provided with a sash 3 is cut in straight vertical lines 10 and horizontal lines 11 with a diamond wheel cutter to form a square opening 13 in which one side is an inserting part 12 into the sash 3. Since the supporting power of the sash inserting part 12 is sufficient to support the cutting reactional force, no special tool, etc., is required. After forming the opening, a force is applied to a part on the side opposite to the sash inserting part 12 of the opening 13 to separate the opening 13 from the sash 3 and form the objective opening. Thereby, the opening can safely and rapidly be formed at an optional position according to this method.