The present invention provides a polypeptide having an intracellular calcium ion indicator function, which contains the following elements (a)-(c): (a) a polypeptide residue consisting of a membrane localization signal sequence; (b) a first fluorescent polypeptide residue; and (c) a second fluorescent polypeptide residue in the order of (a), (b) and (c) from the N-terminal side, wherein one of the two fluorescent polypeptide residues is a donor for fluorescence resonance energy transfer, the other is the corresponding acceptor, and the two fluorescent polypeptide residues are connected with a linker polypeptide residue containing at least one calpain sensitive sequence, thereby to allow fluorescence resonance energy transfer between them.