A digital mixer including a plurality m of signal processing channels for processing an input signal in accordance with first-type parameters and second-type parameters, said digital mixer comprising: a display device capable of displaying graphics; k controls (106, 536, 536-1 to 536-8) to be operated by a user for performing parameter setting, wherein k < m; characterized in that said digital mixer further comprises a first display control section that, in response to a first selection operation by the user, selects k channels from among the m signal processing channels, and causes said display device to display a channel strip screen (600-1 to 600-8) displaying a plurality of parameters for each of selected k channels, wherein k second-type parameters, of a function common to the k channels, among the parameters displayed on the channel strip screen, are in a selected state and parameters other than the k second-type parameters are in a non-selected state; a second display control section that, in response to a second selection operation to select a desired one of the parameters displayed on the channel strip screen by the user, opens a parameter window (801, 1030, 1100) overlaying the channel strip screen and related to the selected parameter, said parameter window displaying, for each of selected k channels, parameters related to the selected parameter, wherein, (i) when the second selection operation selects a first-type parameter, the second display control section opens the parameter window related to the selected first-type parameter, (ii) when the second selection operation selects a second-type parameter in the non-selected state, the second display control section brings k second-type parameters at the same position as the selected second-type parameter into the selected state without opening the parameter window and (iii) when the second selection operation selects a second-type parameter in the selected state, the second display control section opens the parameter window related to the selected second -type parameter; an assignment section that, (i) when no parameter window is opened over the channel strip screen, assigns each of the k second-type parameters in the selected state, displayed on the channel strip screen, to each of the k controls (106, 536, 536-1 to 536-8), and (ii) when a parameter window is opened over the channel strip screen, assigns each of k parameters, displayed on the parameter window, to each of the k controls; and a parameter control section that, in response to a setting operation on any of the k controls (106, 536, 536-1 to 536-8) by the user, controls a value of the parameter currently assigned to the operated control (106, 536, 536-1 to 536-8). a display device capable of displaying graphics; k controls (106, 536, 536-1 to 536-8) to be operated by a user for performing parameter setting, wherein k < m; characterized in that said digital mixer further comprises a first display control section that, in response to a first selection operation by the user, selects k channels from among the m signal processing channels, and causes said display device to display a channel strip screen (600-1 to 600-8) displaying a plurality of parameters for each of selected k channels, wherein k second-type parameters, of a function common to the k channels, among the parameters displayed on the channel strip screen, are in a selected state and parameters other than the k second-type parameters are in a non-selected state; a second display control section that, in response to a second selection operation to select a desired one of the parameters displayed on the channel strip screen by the user, opens a parameter window (801, 1030, 1100) overlaying the channel strip screen and related to the selected parameter, said parameter window displaying, for each of selected k channels, parameters related to the selected parameter, wherein, (i) when the second selection operation selects a first-type parameter, the second display control section opens the parameter window related to the selected first-type parameter, (ii) when the second selection operation selects a second-type parameter in the non-selected state, the second display control section brings k second-type parameters at the same position as the selected second-type parameter into the selected state without opening the parameter window and (iii) when the second selection operation selects a second-type parameter in the selected state, the second display control section opens the parameter window related to the selected second -type parameter; an assignment section that, (i) when no parameter window is opened over the channel strip screen, assigns each of the k second-type parameters in the selected state, displayed on the channel strip screen, to each of the k controls (106, 536, 536-1 to 536-8), and (ii) when a parameter window is opened over the channel strip screen, assigns each of k parameters, displayed on the parameter window, to each of the k controls; and a parameter control section that, in response to a setting operation on any of the k controls (106, 536, 536-1 to 536-8) by the user, controls a value of the parameter currently assigned to the operated control (106, 536, 536-1 to 536-8).