Wearable robot arm systems have been proposed that enable various tasks that humans can not do with their only two natural arms. However, many of these do not have a somatosensory feedback system so that users have to confirm the robot's state by looking every time. This active confirmation can often be a 'switching cost' of task and reduce the multitasking efficiency performed by natural arms and additional robotic arm. In this paper, we focused on the proprioception among the somatosensation, and propose a 'Position haptic belt' which is a new biofeedback system for a wearable robot arm. The system represents the three-dimensional position of a robot hand by the vibrator's placement and vibration frequency. The prototype we developed is able to identify the robot hand position with an accuracy of about 10 cm, and it is used to conduct a case study to evaluate the usability in multiple task situation. The result suggested that the effectiveness of the somatosensory feedback information of robot hand position, even for the wearable robot arm which can be easily confirmed its state on user's own eyes.