Postglacial relative sea-level change of the Okayama Plain Area has been analyzed. Former sea-levels were recognized by using peat beds and marine organisms which are also available for 14C dating. Peat beds were rhought to be formed under a stable sea-level condition. 14C ages of the analyzed peat beds were seprated by 4 groups ; 30～18 ka BP, 11～8 ka BP, 6～4 ka BP and 3～2.5 ka BP, indicate 4 stable sea-level periods. 2 transgressive events (18～11 ka BP and 8～6 ka BP) and one regressive event (3.5?～3 ka BP) were also inferred.