Industrial robots contribute greatly to productivity improvement, quality improvement, and cost reduction at production sites. Among industrial robots, robots that can cooperate with humans without safety measures and have a low risk of serious injury in case of contact with humans are called cooperative robots. In the manufacturing field, there is a growing demand for production lines in which humans and robots can coexist and cooperate, rather than production lines in which robots perform all processes independently. This study aimed to pioneer the behavior of cooperative robots that use the same tools as humans. Taking advantage of the fact that the body of the robot used in the study was made by a 3D printer, we investigated the effect of arm length on the motion of the robot through the performance of an electronic musical instrument called “Otamatone”. From the performance results, it was found that performance accuracy worsened when the arms were extended. Assuming that an increase in the torque borne by the arm affects playing accuracy, we conducted the experiment again using a power assist with a passive electric balancer and found that torque was not the main factor in the deterioration of playing accuracy.