A seal hardening furnace is presented in which seal lines in a liquid crystal display panel are hardened. The seal hardening furnace includes a cassette having a rack bar structure. The rack bar structure has rack bars for supporting the substrate along one direction and rack bar supports at ends of the rack bars that support the rack bars. The rack bars have air discharge openings therein. An air injecting passage is connected to the rack bar supports. An air supply unit supplies air through the air injecting passage and through the discharge openings to support the substrate thereon.