Recent SN surveys are discovering SNe with very high luminosity (brighter than -21 mag) and they are called superluminous SNe (SLSNe). At first, I will introduce SLSNe and their observational varieties. One idea to explain the huge luminosities is the collision of dense CSM and SN ejecta. If SN ejecta is surrounded by dense CSM, the kinetic energy of SN ejecta is efficiently converted to radiation energy, making them very bright. I will show our results of numerical modeling of such collisions and their light curves. From the light curve modeling, we obtain the properties of CSM and mass loss of the progenitors of SLSNe and some constraints on the progenitors. I also shortly introduce the transient survey planned for Subaru/Hyper Suprime-Cam.