Nondisruptive problem behaviors exist to a large extent in group-oriented Japanese regular classrooms. However, many children remain untreated. We implemented an antecedent-based functional behavioral assessment (FBA) and developed a behavioral support program for a first-grade boy who exhibited inattentive behavior in a Japanese regular classroom. Working collaboratively with the classroom teacher, we assisted the classroom teacher gain whole-class or individual attention before expressing whole-class demands. The child's behavior improved and the teacher evaluated our behavioral collaboration favorably. We continue to use FBA with other children in Japanese classrooms with teachers as main change agents. (Contains 1 figure.)