A traffic light detecting device includes an image capturing unit configured to repeatedly capture images in a travelling direction of a vehicle to obtain a series of multiple images and a traffic light detecting unit configured to detect traffic lights from the images. The traffic light detecting unit detects phase information of an electric power system used in an area around the vehicle including the traffic lights from a cycle of a luminance variation in the series of multiple images and extracts from the images a synchronized pixel with a luminance which varies in synchronization with an alternating current cycle of electric power supplied to the traffic light by using the phase information of the electric power system. The traffic light detecting device judges from the synchronized pixel whether or not the traffic light is present.