An input apparatus including an optical mouse module and a mouse pad module is provided. The optical mouse module includes a light source and an optical sensor. The light source is adapted to emit a light beam. The optical sensor has a sensible distance. The mouse pad module is disposed on the optical mouse module. The mouse pad module includes a sliding sheet elastically coupled to the optical mouse module. The sliding sheet is adapted to move in a three-dimensional space. The optical sensor and the sliding sheet are arranged in an arrangement direction. An operation method of the input apparatus is also provided.