We measured the straight carapace length (SCL) of 1392 loggerhead turtles of the Japanese aggregation that were incidentally captured between July 2002 and November 2009 by pound nets set near the eastern coast of Cape Muroto, Kochi Prefecture, Japan. Each specimen was categorized as adult, subadult, or juvenile on the basis of their maturity status inferred from SCL. In the present sample, SCL exhibited a unimodal distribution with the mode located in the 740–749 mm class (±SD: 757±67 mm; range: 563–1050 mm). The majority of loggerhead turtles composing the Japanese aggregation was subadult (75.9%). The SCL of smaller specimens suggested that reentery into Japanese waters of individuals once flown to the northeastern Pacific occurs in the later juvenile stage. A unimodal histogram implies that the major size classes in SCL of turtles reentering Japanese waters range from 560–749 mm. A comparison between the Japanese aggregation and the Mexican aggregation indicates that turtles at the later juvenile stage start to migrate westward, taking only a few years to cross the North Pacific to Japanese waters.