A V-LED (20a), B-LED (20b), G-LED (20c) and R-LED (20d) for an endoscope are all driven to apply normal light to an object of interest in a body. An image sensor (48) images the illuminated obj ect and outputs an RGB image signal. A measurement sensor (25) measures a light amount of red light from the R-LED (20d). A light source controller (21) acquires a current value (c) for the R-LED (20d) according to a light amount signal from the measurement sensor (25) and a target light amount signal for the R-LED (20d). A normal color converter (68) and the R-LED (20d) receive a current of the current value (c). Each LUT_Mij in the normal color converter (68) is referred to for outputting a matrix coefficient (Mij) according to the current value (c) of the R-LED (20d). The RGB image signal is converted into a processed image signal by matrix operation according to the matrix coefficient (Mij).
