  We show a wave-particle duality of light and its complementary relation in
the context of the intensity interference measured by intensity correlation
measurement, especially for the case of the second-order intensity interference
observed in the Hong-Ou-Mandel interferometer. Different from the complementary
relation appearing in the interference based on the phase coherence like in the
Young's double-slit interferometer, the complementary relation in the intensity
interference has a gap between classical and nonclassical lights. This reveals
a new nonclassical nature of light where both wave and particle properties are
classically understandable. We further extend the wave-particle duality and the
complementarity to higher-order intensity interferometric situations.
