In order to confirm the gelling mechanism of agarose, the gel point, coil-helix point, gel melting point, and certain physical properties were measured at various concentrations below 5 g dl-1. A phase diagram of the gelling system was obtained by plotting the reciprocals of these transition temperatures and the changing temperatures of the physical properties against the concentration. This diagram is basically quite similar to that for the rod-like polymer systems proposed by Flory. From the qualitative coincidence of both diagrams, it was confirmed that gelation was caused by the phase separation of the helical segment into isotropic and anisotropic solution phases. Characteristic properties of the gel such as turbidity, electric conductivity, specific optical rotation, pore size, and swelling were interpreted by the gel structure based on the gelling mechanism and by the gel composition based on the phase diagram. KEY WORDS Agarose I Gelling Mechanism I Coil-Helix Transition I Phase Separation I Rod-like Polymer I Optical Rotation of Gel I Swelling of Gel I * Presented at SDSJ 28th Symposium on