We visually inspected the light curves of 7557 Kepler Objects of Interest (KOIs) to search for single transit events (STEs) possibly due to long-period giant planets. We identified 28 STEs in 24 KOIs, among which 14 events are newly reported in this paper. We estimate the radius and orbital period of the objects causing STEs by fitting the STE light curves simultaneously with the transits of the other planets in the system or with the prior information on the host star density. As a result, we found that STEs in seven of those systems are consistent with Neptune- to Jupiter-sized objects of orbital periods ranging from a few to $\sim$ $20\,\mathrm{yr}$. We also estimate that $\gtrsim20\%$ of the compact multi-transiting systems host cool giant planets with periods $\gtrsim 3\,\mathrm{yr}$ on the basis of their occurrence in the KOIs with multiple candidates, assuming the small mutual inclination between inner and outer planetary orbits.
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