Pyrene (Py) contained in poly(N-isopropylacrylamide) gel (NIPA gel) that is swollen by water emits excimer-like derived fluorescence. However, as dehydration proceeded with increasing temperature, the luminescence changed to the monomer-derived one. Changes of it occurred continuously and reversibly around the lower critical solution temperature of NIPA gel. Because the gel interior becomes a hydrophobic environment in the shrunken state, the Py molecules can diffuse. However, it gathers like an oil droplet in the swollen gel by water. Reversible change of fluorescence wavelength with phase separation of Py according to hydration of NIPA gel represents an unprecedented type of chromism.