A frame luminance detection unit (15) detects a frame luminance value for each frame. A flicker spectrum detection unit (16) detects spectrum values such as frequency 100 Hz, 200 Hz, 300 Hz from frameluminance values of 512 frames. A luminance estimation unit (17) estimates a luminance value of a light source from the spectrum values. An encoding unit (18) decides a reference frame by using a reciprocal of the estimated luminance value of the light source and detects a motion vector by using the reciprocal as a coefficient of an evaluation function, thereby encoding an image frame.