Occurrence frequency distribution of superflares as a function of the rotation period. The horizontal axis indicates the rotation period of the star derived from photometric data. The vertical axis indicates the number of superflares per star and per year in each period bin. Period bins are defined as follows: P<2.5 days, 2.5≤P<5 days, 5≤P<10 days, 10≤P<20 days, and P≥20 days. Solid line histogram represents the occurrence frequency of superflares with an energy of >1033 erg in each period bin. The dashed down arrow indicates the upper limit of the flare frequency.