PURPOSE: To automate cutting of corners of a sheet paper stack in a book cover making machine and to improve cutting quality and cutting precision.

CONSTITUTION: This device for cutting corners of a sheet paper stack for processing in a book cover making machine 2 is provided with a corner cutting device 3 installed to be parted from a paper supply station of the book cover making machine 2 having cutting blades 6; 25, 25a which can act on respective corners of the sheet paper stack 1, and a pressure device 12 capable of lowering onto the sheet paper stack 1; and a stack elevation table 4 having side stack guides 21-23 movable from a sheet paper stack 1 charging position to a position in the cutting device 3 to eliminate the corners of the sheet paper stack 1 and a paper supply position in the book cover making machine 2.