To productively manufacture a seat cushion pad for a backrest for an automobile which is a large and integrated product like a high back, homogeneous in whole, and has a good strength and durability by making the best use of existing facilities.

A seat cushion pad A for a high back backrest comprising a main body 4 of a cushion having a seat back part 1 and bulge parts 3, 3 on the both sides of the seat back part, and an upper cushion body 6 forming a head rest part 5 and having a virtual height H of the whole of 840 mm or higher is integrally formed by injecting and foaming a foaming material in a single foaming space. To manufacture such a large seat cushion pad A, an upper mold of the foaming molds is shaken and opened upto an angle in the range of 110 to 120 degrees to a lower mold, whereby the foaming material can be evenly injected in the whole foaming space with an existing injector.