When an optional unit (40, 50) is mounted, a video controller (27) serving as main control unit sends various control commands to a supervising controller (101) through an interface (114). The supervising controller (101) controls an engine controller (26) through an interface (29) while controlling the optional unit (40, 50) in accordance with the received control commands, thereby recording an image. When no optional unit is mounted (when the optional units 32, 40, 50 are absent), the video controller (27) directly controls the engine controller (26) through an interface (28), thereby recording an image. With the above arrangement, the supervising controller (101) serving as a control unit associated with optional units can be attached/detached.