Kego Fault is a 50km-long northwest-trending left-lateral active fault in north Kyushu, Japan. 2005 West off Fukuoka Prefecture Earthquake was generated from the north part of its submarine extent. We conducted single-channel seismic profiling and coring of sediments in Hakata bay in order to reveal Holocene paleoseismicity for seismic risk evaluation of the unbroken southern part of the fault, on which highly-populated Fukuoka city is developing. The active fault traces in the bay extend continuously from land, and keep same mode and sense of faulting shown on land as left-lateral strike slip with downthrown to the east. Our high-resolution seismic profile records show consistent vertical displacement down to the east along the fault traces in the Holocene sediments. Horizontally deposited layers below 2.1m from the bay floor were successively faulted, and the layers on down-thrown side between 2.1 to 2.6m were faulted by 0.3m, and 3.8 to 6.2m by 0.6m, and the acoustic basement by 3m, suggesting that there were three or more seismic events during Holocene. We collected four piston-core samples from both sides of the fault traces. The gray colored fossiliferous Holocene mud shows no evidence of hiatus in the 8m-long cored layers on the downthrown side. The carbon-14 dates of molluscs and echinoderms fossils suggest that the latest two events took place between 4,500 yBP to 4,000 yBP, and 8,500yBP to 6,500 yBP, respectively. Considering the interval of the events and the timing of the latest event, we positively warn that the southern half of Kego Fault has high probability in generating a large earthquake anytime in near geological future.