Mixer includes first and second displays each capable of displaying a pop-up screen that simultaneously displays pieces of information of eight channels. Once a display instruction for displaying a pop-up screen is received, it is ascertained whether or not the pop-up screen currently instructed to be displayed can be displayed on both of the first and second displays. If the pop-up screen can be displayed on only one of the displays, the currently instructed pop-up screen and one-screen channel selection switch are displayed on the one display together with. If, on the other hand, the pop-up screen can be displayed on both of the displays, the instructed pop-up screen are displayed on in a two-screen format on individual ones of the displays, and a two-screen channel selection switch is displayed on each one of the pop-up screens.