An object of the present invention is to provide a fluorescent protein derived from organisms other than Aequorea victoria and having a novel primary structure. According to the present invention, there is provided a fluorescent protein derived from Halcurias sp. L, which has the following properties: (1) the excitation maximum wavelength is 494 nm, and the fluorescence maximum wavelength is 506 nm; (2) the molar absorption coefficient at 494 nm is 94600 M.sup.-1 cm.sup.-1; (3) the quantum yield is 0.65; and (4) the pH sensitivity of the fluorescent property is low in the range between pH 4 and pH 11.