The purpose of the present invention is to provide detection methods of Notch signaling activation for detecting the activation of the Notch signaling in living cells simply and conveniently. The expression of a fluorescent protein Venus in a transgenic cell into which a vector having the fluorescent protein Venus gene which is controlled by the wild-type Hes-1 gene promoter has been introduced is compared with the expression of a fluorescent protein Venus in a transgenic cell into which a vector having the fluorescent protein Venus gene controlled by a mutated Hes-1 gene promoter which is not controlled by an activated Notch protein has been introduced, and a transgenic cell in which a signal by the expression of Venus introduced by the vector having the wild-type Hes-1 promoter is observed and in which a signal by the expression of Venus introduced by the vector having the mutated Hes-1 gene promoter which is not controlled by the activated Notch protein is not observed is identified.