  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.
