Stochastic computing [1] represents data as random bit sequences, enabling complex arithmetic with simple logic. However, it suffers from high latency and limited precision. Deterministic stochastic computing [2] uses shorter deterministic bit sequences, reducing latency and improving accuracy. Moreover, single-flux quantum (SFQ) circuits, known for their ultra-high speed, are promising candidates for implementing stochastic computing hardware [3]. We proposed a superconducting deterministic adder based on SFQ circuits. Compared with the conventional SFQ stochastic adder that relies on a multiplexer (MUX) controlled by random bit sequences [4], our design uses only an AND gate, an XOR gate, and a toggle flip-flop, significantly lowering area overhead. Additionally, the deterministic adder requires no random control signal, reducing the number of random number generators and avoiding precision loss due to probability deviations of the control signal. We fabricated this adder using a 10 kA/cm² Nb four-layer superconducting process and verified its high-speed operation up to 35.4 GHz. For a 1024-bit sequence, our adder achieves a 32× reduction in root-mean-square error, a 49.0% reduction in circuit area, and a 28.1% decrease in Josephson junction count compared to the conventional MUX-based design.