It is intended to provide a fluorescent protein having a novel primary structure originating in an organism other than Aequorea coerulescens. Thus, a fluorescent protein originating inHalcurias sp. L which has the following properties is provided: (1) having a maximum excitation wavelength of 494 nm and a maximum fluorescent wavelength of 506 nm; (2) having a molar absorptio n coefficient at 494 nm of 94600 M-1~cm-1~; (3) having a quantum yield of 0.65 ; and (4) having a low pH-sensitivity of the fluorescent properties in a pH range of 4 to 11.
