PROBLEM TO BE SOLVED: To provide a chopper phase management device that can properly assist work of chopper folding by managing the chopper phase.SOLUTION: A chopper phase management device 1 calculates a print speed for operating a web rotary offset press, and calculates a chopper phase from a rotation signal of a folding cylinder 93 and a driving signal of a chopper blade 101. Then, an approximate curve obtained by approximating the chopper phase with the print speed from these data obtained in the production of good printed matter is calculated, and a speed following curve is created for every print condition. Then, the chopper phase management device 1 pre-sets the speed following curve according to the print condition to the web rotary offset press during operation of the web rotary offset press, and the web rotary offset press sets the chopper phase based on the speed following curve. Also, the chopper phase management device 1 performs warning when the chopper phase during the operation of the web rota
