Some ionic surfactant/water systems undergo the coagel-gel phase transition. Both coagel and gel phases consist of alternating layers of bilayer membranes of surfactants and water. The hydrocarbon chains are in a crystalline state, and thickness of water layer is in the order of 10 Angstrom in the coagel phase. Upon the transition from coagel to gel, the hydrocarbon chains become able to rotate around the chain axis, and the thickness of water layer discontinuously increases to the order of 1000 Angstrom. We present a statistical mechanical theory to explain this phase transition. The mean field Gibbs free energy of the system is written as a function of three-order parameters which are concerned with the rotational motion of the hydrocarbon chains, the degree of dissociation of counterions of the surfactant molecules, and the distance between neighboring membranes. The area occupied by one hydrophilic head group on the membrane surface in the rotating state of the hydrocarbon chain is assumed to be larger than that in its fixed state. The theory successfully explains the order-disorder phase transition of the hydrocarbon chains and the simultaneous separation of bilayer membranes. The expansion in the area of one molecule leads to a coupling between three-order parameters. The theoretical calculation predicts that the strength of this coupling results in the three types of phase transition : (1) coagel/gel phase transition, (2) pseudogel (the hydrocarbon chains are in crystalline state, and the membranes separate)/gel phase transition, (3) coagel/pseudocoagel (the rotation of hydrocarbon chains around the chain axis is released, and the membranes do not separate) phase transition.