A plurality of pot support members each for detachably supporting the pot are connected, and an endless track is generated for guiding the pot support members driven by sprocket members. Each of the pot support members is supported with a pair of pins so that the pot support members are connected into a chain-like formation. The pitch between the pair of pins for supporting the pot support member is made substantially equal to the pitch between the pins of the adjacent pot support members so as to allow the resultant track to meander both inward and outward.