A thermoelectric conversion unit includes first and second cases including first and second recesses that define first and second flow passages and have openings, first and second substrates, a plurality of thermoelectric devices in contact with inner surfaces of the first and second substrates, a connector for joining the first and second cases, first and second fins protruding from the first and second substrates in the first and second flow passages. The first and second cases include first and second opposed portions opposed to the first and second substrates outside the first and second recess, respectively. A spacer for setting distance between the first opposed portion and the second opposed portion is provided. The spacer sets the distance between the first and second opposed portions to be longer than distance between the outer surface of the first substrate and the outer surface of the second substrate.