A level shifter converting a binary signal having a first potential and a second potential into a signal having the first potential and a third potential, and a signal processing circuit using the level shifter are provided. The first potential is higher than the second potential. The second potential is higher than the third potential. The potential difference between the first potential and the third potential may be more than or equal to 3 V and less than 4 V. The level shifter includes a current control circuit which generates a second signal for operating an amplifier circuit for a certain period in accordance with the potential change of the first signal which is input to the amplifier circuit. The output of level shifter is input to a gate of an N-channel transistor whose threshold voltage is lower than 0 V.