On a digital mixer, each channel is given a channel definition defining whether the channel is to be used individually or to be used as a group along with a certain channel. When a channel strip to which a user desires to assign and a channel which the user desires to assign are selected, it is determined whether the selected channel is a channel to be used individually or to be used as a group. When it is determined that the selected channel is to be used as a group, channels belonging to the group are assigned to the selected channel strip so that the user can concurrently control respective channels' values of a parameter by use of an operating element provided on the selected channel strip.