The fluorescence behavior of isotactic polystyrene (iPS) in cis- and trans-decalin and mixed decalin solutions with some concentrations was measured continuously between 373 and 203 K. A hysteresis loop is observed for the temperature dependence of the fluorescence intensity ratio of excimer to monomer, I-E/I-M, in all the iPS solutions. In particular, I-E/I-M of iPS changes dramatically at temperatures where gelation takes place at high concentrations, and transition from random-coil to helix structure of the iPS chain takes place in dilute solution. It is concluded from the fluorescence data that the local motion of the iPS main chain is restricted in the helical structure. Consequently, the physical gelation of the iPS-decalin system is shown to take place by the formation of the nodes that are kinds of microcrystals made up of oriented helical rods and can be a cross-linking point of the gel network. The formation of helical rods appears to be urged favorably near nodes. Moreover, the time change of the iPS chain structure is first and directly demonstrated by our fluorescence method; it changed with time for 2 weeks at the transition temperature. The time profile of iPS in cis-decalin was found to be different from that in trans-decalin and mixed decalin, and the difference is due to the interaction between the helical structure of the iPS chain and the decalin isomer. The structure of the iPS chain is also discussed in the present paper.