Provided is an apparatus for computing a T-function based Stream Cipher (TSC)-4 stream cipher. The apparatus includes: two T-function units; and a nonlinear filter for receiving bits output from the two T-function units and generating an 8-bit output sequence per clock. Each of the T-function units includes: a first register for storing an internal state value of the lower N bits; an N-bit internal state updater for updating the internal state value of the lower N-bits stored in the first register; an intermediate result register for storing an intermediate result value output from the N-bit internal state updater; a second register for storing an internal state value of the upper M bits; and an M-bit internal state updater for updating the internal state value of the upper M bits stored in the second register using the value stored in the intermediate result register.