In this paper, we introduce a practical contact observer which allows detecting a contact on the body surface of a humanoid robot. Our method estimates the error force due to undesired contact. Then, contact detection (yes/no) is performed together with the estimation of the contact force and location. By comparing the real robot state according to sensory data with the desired state computed from the contact-free dynamics equation, we can estimate the external force applied to an unknown link. The link in contact is then detected by tracing the torque error along the robot's kinematic chain. Once the link in contact is determined, we can estimate the point where the contact force was applied by using the 3D geometric mesh model of a robot. The proposed method is validated through simulation and experiments using the humanoid robot HRP-4.