Various sensor can be introduced into the robot hand such as proximity sensor; detecting an object position in non-contact using reflection of an electric field or light. Introducing tactile and proximity sensor into robot hand, it can obtain great sensory ability compared with human. On the bases of this concept, we developed "Slip and Proximity Integrated Fingertip Sensor" which has Net-Structure Proximity Sensor and Slip Sensor. In this paper, we focus on Net-Structure Proximity Sensor output in a contact state. We discuss a possibility that object stiffness can be distinguished by Proximity Sensor output in a contact state.