A polarization conversion device converts a polarization state of a light which is input to a first waveguide, that is, TE/TM mode of the light, to output it from the first waveguide. The polarization conversion device includes: a mode converter that performs the inter-conversion of TE/TM modes of the light which is input to the first waveguide; and a polarization separator that receives the light passed through the mode converter and separates the received light into a first light, TE/TM of which mode has been converted by the mode converter and a second light, TE/TM of which mode has not been converted, to output the first light to the first waveguide.