The　self-propulsion　of　a　Janus　particle　suspended　in　a　dilute　gas　at　equilibrium　is　investigated　in　the　free　molecular　regime.　The　Janus　particle　consists　of　two　hemispheres　with　different　momentum　accommodation　factors;　the　particle　and　the　surrounding　gas　are　held　at　different　constant　temperatures.　Based　on　the　gas　kinetic　theory,　we　calculate　the　particle＇s　self-propulsion　and　drag　force.　We　conclude　that　self-propulsion　occurs　only　under　the　condition　that　the　particle　is　hotter/colder　than　the　suspension　gas,　and　the　self-propulsion　force　is　proportional　to　the　difference　of　the　momentum　accommodation　factors　and　directed　along　the　symmetry　axis.　The　drag　force,　instead,　is　corrected　by　a　term　proportional　to　the　average　of　the　momentum　accommodation　factors.　Our　analytical　results　are　confirmed　by　numerical　Monte　Carlo　simulations.