According to one embodiment, a solid state image sensor has a photoelectric conversion element array, a light collecting optical element array, wavelength-selective elements and a reflecting unit. Wavelength-selective elements pass light of the color which is to be detected, and reflect other colors. The reflecting unit further reflects the light that has been reflected by the wavelength-selective elements. The cell includes photoelectric conversion elements for three different light colors. A microlens serving as a light collecting optical element is arranged corresponding to the cell. The reflecting unit includes at least a first reflecting surface and a second reflecting surface. The first reflecting surface faces the wavelength-selective elements. In every cell, the second reflecting surface is enclosed between wavelength-selective elements and the first reflecting surface.