The dynamics of polystyrene microgel (PSMG), i.e., a probe particle of highly cross-linked polystyrene latex spheres with radii 20.7 and 284 nm, during the sol-gel transition of poly(gamma -benzyl-L-glutamate) (PBLG) dissolved in N,N-dimethylformamide (DMF) solution has been investigated by measurements with the dynamic light scattering technique. All the intensity time correlation functions (ICF) measured from PSMG dispersed in PBLG matrix approximately decayed single exponentially. First, it was confirmed that the diffusion coefficients of PSMG at the sol state of 298.8 K obeyed the stretched exponential relation as a function of concentration of PBLG-DMF solutions. Second, the initial amplitude and the normalized variance of ICF, and the diffusion coefficient of PSMG were altered considerably before and after sol-gel transition with decreasing temperature. The initial amplitude and the normalized variance had a minimum and a maximum at the gelation threshold temperature, respectively. It was also found that PSMG almost free-diffused even at the gel state, as well as the sol state, regardless of low temperature. These facts may strongly support that the resulting PBLG gels induced through microphase separation evidently had a porous and bicontinuous structure at the "swollen gel state," which corresponds to the results observed by a transmission electron microscope.