Petrogenetic grids, which are production of the Schreinmakers’ analyses of multisystems, are very useful to metamorphic petrologists. The author considers metabasite metamorphism in lower-grade of the Sanbagawa metmorphic belt as a thermodynamic system. The system contains lawsonite, epidote, pumpellyite, diopside, actinolite, glaucophane and chlorite with excess albite, quartz and water and can be formed by pseudo-three system components which are A, C and F. There are 21 invarriant points and 35 univariant curves in the system. To reduce the number of the possible petrogentic grids by half, it is proposed that the slopes of the univariant curves should be calculated by using a series of approximations. To construct petrogenetic girds, an invariant points circuit was proposed. The invariant points circuit is composed of invariant points in homotypical relation. The invariant point of inflow is situated at the highest pressure among invariant points constituting petrogenetic grids. Thus, petrogenetic grid starts from an outflow invariant point and ends to an inflow invariant point. With the proposed method seven independent sets of invariant points are obtained. Therefore, with projection on the P-T diagrams, seven petrogenetic grids are constructed in the system