To provide a new electric potential-sensitive dye capable of measuring an action potential of a nerve cell, a cardiac muscle cell, etc., fast and sensitively.

This electric potential-sensitive dye comprises a rhodacyanine derivative or its salt. The rhodacyanine derivative is represented by formula (I) or (II) (wherein R represents an alkyl group which may have a substituted group or an aryl group which may have a substituted group; A and B represents each independently a five- or six-membered ring containing at least one heteroatom or a condensed ring thereof which may have a substituted group; L1 and L2 represent each independently a methine group or a substituted methine group; L1 and a part of A may combine to form a five- or six-member ring when L1 is the substituted methine group; L3 represents a methine group, a substituted methine group or a nitrogen atom, and L3 and a part of B may combine to form a five- or six-membered ring when L3 is the substituted methine group).