Poly(N-isopropylacrylamide) solution, which is a representative thermoresponsive polymer, exhibits a phase separation with a formation of polymer-rich microparticles due to dehydration and aggregation of the polymer chains above a lower critical solution temperature (LCST). However, little is known about the details of polymer concentration in the particle. We have developed optical tweezers combined with confocal Raman microspectroscopy to analyze a hydration structure in a single polymer rich particle. A focused near-infrared laser beam produced the microparticle at the focal point due to an optical force and a photothermal effect. In this study, we investigated that molecular weight dependence of the polymer concentration in the polymer-rich particle by means of Raman microspectroscopy.