16 stands of Pinus plain forest in Chiba and Ibaraki Pref. were investigated, and JACCARD's similarity indices among them according to the floras of floor layers were calculated. Through the factor analysis by the Direct Varimax Method based on the JACCARD's similarity indices, the factors of plagiosere effected by underscrubbing and orthosere were estimated. Ordinating the 16 stands along the two factor axes by the factor loadings, the 16 stands were divided into three groups, i.e., group A: well underscrubbed stands; group B: slightly underscrubbed stands or stands without underscrubbing for several years; and group C: stands without underscrubbing for over 10 years. Comparing the life-form spectra according to the number of species among A, B, and C, the percentages of microphanerophytes and geophytes in the RAUNKIAER's life-form system, erect forms and liane forms in the NUMATA's growth form system, and D₂ (zoochore) in the NUMATA's disseminule form system increased in the order of A, B, C, i.e., along the succession of Pinus stage. On the contrary, the percentages of chamaephytes and hemicryptophytes in the RAUNKIAER's life-form system, tufted growth and partial rosettes in the NUMATA's growth form system, and D₄ (bolochory) in the NUMATA's disseminule form system decreased in the same order, i.e., along the succession of Pinus stage.