Mobile phones are in a trend of having thinner configuration due to emphasis on the design. Thus, in order to meet this demand, the present invention aims to thin the key sheet portion as much as possible. A thin key sheet and a thin key unit incorporating the thin key sheet are configured by providing on its surface a key top 2 made of a metal or a resin with indications such as letters, symbols, graphics or the like indicating the functions of keys are formed on the front surface or the back surface thereof, and switch elements 4 for turning ON/OFF of an electric circuit by the pressing of the key top, together with a printed circuit board 5 on which a circuit pattern involved therewith is formed; and by adhering and fixing the key top 2 and the printed circuit board 5 together via a cushion gasket 3 having appropriate elasticity and a filler layer 14 disposed between the key top and the printed circuit board.