The authors examined the effect of interdependent group-oriented contingencies on sitting behaviors of elementary students at the beginning of the regular classes. The study was conducted in a 2nd-grade classroom of a public elementary school. Participants were 25 children（14 boys, 11 girls）. The target behavior was described as follows: “I will sit down as soon as the chime rings.” The authors observed and recorded 42 sessions including the first 1-17 baseline phase, the following 18-34 intervention phase, and the final 35-42 follow-up phase. After the 17th session, the target behavior was set for all the children for sessions 18-34, and the class teacher announced that a medal sticker with a popular cartoon character would be attached to the class goal poster each time they all sat down as soon as the chime rang, and promised that when they achieved this 15 times, a fun event would be held as a reward. All through the intervention period the frequency of sitting behavior increased and the completion time was shortened. Application of interdependent group-oriented contingencies was effective in improving the sitting behavior of the children.