Abstract: A wavelength tunable filter selectively transmitting light of any desired wavelength within the range of 450 nm to 1100 nm without using a band pass filter is disclosed. The wavelength tunable filter comprises a plurality of polarizers and a plurality of units between the adjacent polarizers on a common optical axis in a housing. Each unit has, in order from light incident side, a quartz birefringent plate, an ultra-wide band ¼ waveplate and an ultra-wide band ½ waveplate. The ultra-wide band ½ waveplate is accommodated in a cylindrical rotatable body and it is rotated by a rotating mechanism.