Because touch is a mechanical interaction between the skin and an object, unconstrained tactile sensing is a challenging issue. In this research we studied spatial transparency, a new concept in unconstrained tactile sensing that allows collection of tactile information without affecting the touch conditions. Especially, we aimed at developing a spatially transparent tactile sensor that detects touch information with high dynamic range from any part of the body. For the development of such sensors, this report focuses on the electromechanical properties of the skin, particularly elasticity and conductivity, and proposes a novel tactile sensor based on measurement of the electrical contact impedance between the skin and an object. The measurement of skin contact impedance is achieved using a grounding electrode and two signal electrodes attached to any part of the body. We investigated the relationship between touch force and output of the proposed sensor using a force sensor. The experimental results indicate that the proposed sensor functions adequately as an unconstrained tactile sensor, and confirm that the proposed sensing system has an excellent dynamic range.