To realize the GW-class power source for Microwave Rocket, the cost reduction of gyrotrons is one of the solutions to lower the threshold to realize the beam station. In this study, a sub-MW-class gyrotron which adapted a smaller bore diameter of the superconducting magnet was designed and developed. The output power, pulse duration, and frequency are respectively 600 kW, $100\ \mu\mathrm{s}$, and 94 GHz. The electron current is driven by a charged capacitor bank and in-house IGBT switches. The gyrotron tube was aligned with the superconducting magnet to achieve high electric efficiency.