A method for performing phase identification of a cell cycle, the method comprising the steps of: visualizing, at least two or more gene-expression products whose amounts in a cell change in a cell-cycle dependent manner; and detecting in vitro the at least two or more gene-expression products so as to distinguish a proliferation phase of the cell cycle from a resting phase of the cell cycle, wherein: as the at least two or more gene-expression products, at least (i) a first gene-expression product whose amount increases in a G1 phase and decreases in an S/G2/M phase and (ii) a second gene-expression product whose amount decreases in the G1 phase and increases in the S/G2/M phase are used in one cell; the first gene-expression product is the partial fragment of Cdt1, which partial fragment of Cdt1 is remaining of Cdt1 from which a Geminin binding site is excluded; the second gene-expression product is the partial fragment of Geminin, which partial fragment of Geminin is remaining of Geminin from which a Cdt1 binding site is excluded, and the first gene-expression product and the second gene-expression product being labeled by the markers different from each other, so as to enable the first gene-expression product and the second gene-expression product to be visualized and detected, wherein the first gene-expression product is the partial fragment of Cdt1, which partial fragment of Cdt1 is composed of 30th through 120th amino acids of Cdt1, and wherein the second gene-expression product is the partial fragment of Geminin, which partial fragment of Geminin is composed of 1st through 110th amino acids of Geminin. visualizing, at least two or more gene-expression products whose amounts in a cell change in a cell-cycle dependent manner; and detecting in vitro the at least two or more gene-expression products so as to distinguish a proliferation phase of the cell cycle from a resting phase of the cell cycle, wherein: as the at least two or more gene-expression products, at least (i) a first gene-expression product whose amount increases in a G1 phase and decreases in an S/G2/M phase and (ii) a second gene-expression product whose amount decreases in the G1 phase and increases in the S/G2/M phase are used in one cell; the first gene-expression product is the partial fragment of Cdt1, which partial fragment of Cdt1 is remaining of Cdt1 from which a Geminin binding site is excluded; the second gene-expression product is the partial fragment of Geminin, which partial fragment of Geminin is remaining of Geminin from which a Cdt1 binding site is excluded, and the first gene-expression product and the second gene-expression product being labeled by the markers different from each other, so as to enable the first gene-expression product and the second gene-expression product to be visualized and detected, wherein the first gene-expression product is the partial fragment of Cdt1, which partial fragment of Cdt1 is composed of 30th through 120th amino acids of Cdt1, and wherein the second gene-expression product is the partial fragment of Geminin, which partial fragment of Geminin is composed of 1st through 110th amino acids of Geminin. as the at least two or more gene-expression products, at least (i) a first gene-expression product whose amount increases in a G1 phase and decreases in an S/G2/M phase and (ii) a second gene-expression product whose amount decreases in the G1 phase and increases in the S/G2/M phase are used in one cell; the first gene-expression product is the partial fragment of Cdt1, which partial fragment of Cdt1 is remaining of Cdt1 from which a Geminin binding site is excluded; the second gene-expression product is the partial fragment of Geminin, which partial fragment of Geminin is remaining of Geminin from which a Cdt1 binding site is excluded, and the first gene-expression product and the second gene-expression product being labeled by the markers different from each other, so as to enable the first gene-expression product and the second gene-expression product to be visualized and detected, wherein the first gene-expression product is the partial fragment of Cdt1, which partial fragment of Cdt1 is composed of 30th through 120th amino acids of Cdt1, and wherein the second gene-expression product is the partial fragment of Geminin, which partial fragment of Geminin is composed of 1st through 110th amino acids of Geminin.