We performed laboratory experiments of Rayleigh-Taylor instability of superposed viscous fluids where the upper layer contains denser spherical solid particles. A series of experiments are made by varying the viscosity and the particle diameter, and we measure the growth rate and the wave length of the instability. The instability consists of fine-scaled plumes, which coalesce as they descend. Plumes are observed to form intermittently and the particle layer thins with time, which finally descend as blobs. We find that the growth rate can be explained by using the linear stability analysis for Rayleigh-Taylor instability of viscous fluids, by taking the effective viscosity of particle bed to be 20 times that of the fluid, and scale the thickness of the upper layer by twice the particle diameter. Using this scaling, we find that a partially solidified layer beneath the surface of a lava lake may become unstable by this mechanism.