PURPOSE: To obtain a magnetic disk substrate having high flatness at low cost in massproduction by cutting a rod-state base material with a multiwire saw.

CONSTITUTION: After a prismatic base material (3) having an almost square cross section and made of aluminosilicate glass is fixed on a float glass (4) with wax and then fixed to a work bench (5) connected to a pressing up device (6). Wires (2) are wound on grooves of three multigroove rollers ((1)-1 to (1)-3) and traveled in one direction at a specified speed, while the base material (3) is pressed upward at a prescribed speed with the pressing up device (6) to press the wires (2) to cut the base material (3). During cutting, a specified grinding liquid is supplied by a specified amt. from the upper part of the base material (3). Features of this method are as follows, (a) the base material can efficiently be cut in massproduction with good flatness. (b) times required for cutting and its incidental works are greatly saved. Thus, magnetic disk substrates of high flatness can be produced with good productivity at a low cost.