The present invention provides a flowrate control three-way valve and a temperature control device that further improve the sealability at a seal part, which seals a valve element with respect to a fluid at a low temperature of approximately -85°C in a manner that allows rotation of the valve element, in comparison to the case where a sealing means that seals an end part of the valve element on the driving means side thereof with respect to a valve body in a manner that allows rotation of the valve element, is formed from a synthetic resin having a substantially U-shaped cross section, and is biased in the opening direction by a metal spring member is not provided.　The present invention comprises: a cylindrical valve element that is rotatably disposed within a valve seat of a valve body, and that has an opening part for simultaneously switching a first valve opening from a closed state to an opened state and a second valve opening from an opened state to a closed state formed therein; a driving means that rotationally drives the valve element; and a sealing means that seals an end part of the valve element on the driving means side thereof with respect to the valve body in a manner that allows rotation of the valve element, is formed from a synthetic resin having a substantially U-shaped cross section, and is biased in the opening direction by a metal spring member.