A detection device and a monitoring system are adapted to detect an exercising state of a user. The detection device is adapted to be equipped on a circulation exercise device of a physical training equipment. The detection device has a state detector and a rotation speed detector. The state detector generates multiple signal combinations based on a periodic motion of the circulation exercise device. The rotation speed detector estimates a relative variation between a rotation speed and degrees of the circulation exercise device based on the signal combinations. The relative variation represents the user's exercising state. According to the relative variation, a user can realize whether the exercising state is in balance.