Studies on robotic emotions were mostly focused on facial expressions. However, in human emotion recognitions, biological signals were effectively applied in use. In this paper, we clarified the utility of biological signals as a way to express robotic emotion. We designed facial expressions and set up biological signals (heartbeat, respiration) to apply to the facial LED interface for expressing emotions. The results from two sets of experiment demonstrate that biological signals are closely connected to an arousal level and intensity of emotion. It also can be a distinctive element for emotion expression which is deficient only in facial expression.