Erroneous measurement in distance measurement using a TOF technique is prevented. Intensity-modulated light being modulated with a constant cycle is emitted toward a subject. Reflected modulated light from the subject is received by a light receiving unit, and four types of (first to fourth) detection signals with different phases are obtained. Then, whether a difference between a first sum of the first and third detection signals and a second sum of the second and fourth detection signals is smaller than a set value is determined. If it is the case, a phase difference between the modulated light and the reflected modulated light is detected using the four detection signals. Then, a distance to the subject is calculated using the phase difference. In contrast, for any block in which the difference is not less than the set value, the phase difference is not detected and the distance is not calculated.