The developmental approach enables us to build adaptive robots, and furthermore, to understand the essence of intelligence from the constructivist viewpoint. In this paper, a new design principle for tactile sensors is proposed to investigate and to utilize developmental processes of robots. Based on the design principle, an anthropomorphic fingertip is developed. The fingertip is made of soft material with randomly distributed receptors inside. The robot learns to acquire meaningful information such as the slip and the object texture from the outputs of receptors through interaction with the environment like a human does. Several experimental results are shown to demonstrate its sensing ability and applicability for the developmental approach.