A touch panel using an optical sensor has a simple construction and can accurately detect an input position. An illuminating lights emitted from illuminating means are turned into lights having a high directivity in an X-axis direction and in a Y-axis direction of the prism lens sheet and thereafter enter from side faces of a light guide panel as incident lights. The incident lights advance in the inside of the light guide panel toward opposite side faces while being subjected to a total reflection and are received by the optical sensor arrays. When an input pen or a fingertip touches a surface of the light guide panel, the lights are refracted or absorbed at a touched position and hence, a quantity of received lights at the optical sensor arrays is reduced.