A flow channel switching valve comprising: a valve body having a valve chamber formed therein and having a plurality of inlet/outlet ports (p10, p11, p12) opening to the valve chamber; a valve element rotatably arranged within the valve chamber, the valve element having a flow channel formed therein; a seat member (31, 32) disposed between the valve element and one of the inlet/outlet ports so as to seal a gap between the valve element and the inlet/outlet port; an elastic member (33, 34) disposed between the seat member and the valve body so as to press the seat member against the valve element; and a rotary drive portion adapted to rotate the valve element about a rotation axis, wherein: when the valve element is rotated, a communication state of the plurality of inlet/outlet ports is configured to be selectively switched via the flow channel in the valve element, the valve element has a vertical through-hole penetrating through the valve element in a direction of the rotation axis, the vertical through-hole having engaged therewith a valve shaft that transmits torque of the rotary drive portion to the valve element such that the valve shaft is insertable into the vertical through-hole along the direction of the rotation axis in a mutually non-rotatable manner, the valve shaft is inserted through the vertical through-hole and a fit-insertion hole provided in the valve body, characterised in that a drive gear that forms the rotary drive portion is fixed to a portion of the valve shaft protruding from the fit-insertion hole so that the valve shaft is rotatably supported with respect to the valve body, and the valve shaft includes a lower engagement portion, an upper coupling portion, and an intermediate trunk portion, the lower engagement portion having a shape complementary to that of the vertical through-hole, the upper coupling portion having a smaller shape than that of the lower engagement portion and being adapted to have the drive gear fixed thereto, and the intermediate trunk portion having a smaller shape than that of the lower engagement portion. a valve body having a valve chamber formed therein and having a plurality of inlet/outlet ports (p10, p11, p12) opening to the valve chamber; a valve element rotatably arranged within the valve chamber, the valve element having a flow channel formed therein; a seat member (31, 32) disposed between the valve element and one of the inlet/outlet ports so as to seal a gap between the valve element and the inlet/outlet port; an elastic member (33, 34) disposed between the seat member and the valve body so as to press the seat member against the valve element; and a rotary drive portion adapted to rotate the valve element about a rotation axis, wherein: when the valve element is rotated, a communication state of the plurality of inlet/outlet ports is configured to be selectively switched via the flow channel in the valve element, the valve element has a vertical through-hole penetrating through the valve element in a direction of the rotation axis, the vertical through-hole having engaged therewith a valve shaft that transmits torque of the rotary drive portion to the valve element such that the valve shaft is insertable into the vertical through-hole along the direction of the rotation axis in a mutually non-rotatable manner, the valve shaft is inserted through the vertical through-hole and a fit-insertion hole provided in the valve body, characterised in that a drive gear that forms the rotary drive portion is fixed to a portion of the valve shaft protruding from the fit-insertion hole so that the valve shaft is rotatably supported with respect to the valve body, and the valve shaft includes a lower engagement portion, an upper coupling portion, and an intermediate trunk portion, the lower engagement portion having a shape complementary to that of the vertical through-hole, the upper coupling portion having a smaller shape than that of the lower engagement portion and being adapted to have the drive gear fixed thereto, and the intermediate trunk portion having a smaller shape than that of the lower engagement portion. when the valve element is rotated, a communication state of the plurality of inlet/outlet ports is configured to be selectively switched via the flow channel in the valve element, the valve element has a vertical through-hole penetrating through the valve element in a direction of the rotation axis, the vertical through-hole having engaged therewith a valve shaft that transmits torque of the rotary drive portion to the valve element such that the valve shaft is insertable into the vertical through-hole along the direction of the rotation axis in a mutually non-rotatable manner, the valve shaft is inserted through the vertical through-hole and a fit-insertion hole provided in the valve body, characterised in that a drive gear that forms the rotary drive portion is fixed to a portion of the valve shaft protruding from the fit-insertion hole so that the valve shaft is rotatably supported with respect to the valve body, and the valve shaft includes a lower engagement portion, an upper coupling portion, and an intermediate trunk portion, the lower engagement portion having a shape complementary to that of the vertical through-hole, the upper coupling portion having a smaller shape than that of the lower engagement portion and being adapted to have the drive gear fixed thereto, and the intermediate trunk portion having a smaller shape than that of the lower engagement portion.