In the Solar System, four giant planets, Jupiter, Saturn, Uranus, and Neptune have planetary rings. However, around exoplanetary systems, nobody had found an observational clue to such ring systems even though the Kepler spacecraft already has the sensitivity to tiny signatures of rings in transit light curves. To make full advantage of the Kepler data, we developed a precise scheme for modeling transit light curves of ringed planets. Up until now, we have searched for rings around long-period planets like Saturn as well as relatively short-period planets with high signal-to-noise ratios. I will report results obtained so far including the discovery of possible ringed planet and constraints on ring size.