Suiguchi was born in Kasuga, a member of the so-called Matsuzaka Generation, and raised in Ōnojō, Fukuoka. He began playing baseball in the fourth grade for the Onojo Little League baseball club while attending Onojo Municipal Elementary School as an outfielder, being converted to first base the following year. He became a pitcher in the sixth grade, leading his team to the Round of 16 in the national tournament as their ace. He led the city's Onojo Guts to the finals of the national junior tournament while at Ōno Municipal Junior High School.