PURPOSE: To obtain an information recording substrate with superior quality and productivity by forming an annular projection on one of opposed projection faying surfaces of two disks, and joining the two disks by an ultrasonic welding method.

CONSTITUTION: Annular projections 14 and 15 in a triangular shape are formed by injection molding at the nearly center parts of the projection faying surfaces 4 and 5 of the opposed disks 1 and 11 of an information recording substrate. The disks 1 and 11 are centered by using the pin 17 of a discoid base 16. The disks 1 and 11 are fixed between an ultrasonic wave horn 18 and the base 16 and the annular projections 14 and 15 are fused with an ultrasonic wave to weld the disks 1 and 11 together. Consequently, the information recording substrate with superior quality and productivity is obtained.