This study examines how prior information about a partner affects the sense of joint agency during a collaborative task. Previous research suggests that perceiving a partner as human enhances this sense. However, the effects of prior instructions remain unclear. We designed a 2 × 2 factorial experiment in which participants were told their partner was human or a program, while their actual partner was either human or a program. In the experiment, each participant and their partner jointly controlled a cursor to trace a circle. Additionally, we measured the sense of agency to compare it with joint agency. The data revealed that the instructional factor only influenced the sense of joint agency, while the actual partner factor solely affected the sense of agency. These findings suggest that beliefs about the partner prior to interaction influence the sense of joint agency, offering insights into the cognitive processes underlying collaborative actions.