This paper presents a design of ultra-high-speed, low-power arithmetic unit that supports variable bit-width operations with the single flux quantum (SFQ) technology. Because of the high-speed nature of superconductor devices, we can achieve extremely high power-performance efficiency that cannot be achieved by state-of-the-art CMOS devices. To implement the complex function to support the variable bit-width feature, we introduce a novel circuit architecture to maintain the high-speed operation over 50GHz. Our prototype chip design successfully demonstrate 53.5GHz 1.59mW operations.