To reduce power losses due to oil flows in aeroengine gearboxes, the oil flows should be visualized and measured. In this study, we develop a flow visualization borescope that qualitatively visualizes oil flows along with a two-phase flow probe that quantitatively measures the oil/air ratio and flow velocity. The flow visualization borescope comprises a 16-mm-diameter pipe. Within the pipe, an air purge passage for removing oil mist and a borescope are integrated with an illumination laser and optical lenses, enabling clear, high-speed photography. The two-phase probe consists of a 5-mm-diameter pipe with a 1-mm-diameter measurement hole and an internal pressure adjustment pipe. The borescope and flow probe were demonstrated using a shrouded spur gear with a peripheral speed of 100 m/s and oil supply of 20 l/min. Flow visualization at 30,000 fps revealed that oil outflow from the shroud opening spreads turbulently over the entire width of the opening. Measurements of the oil/air ratio and flow velocity using the two-phase flow probe revealed a thin oil-rich layer on the shroud wall and showed that the flow speed is lower than the gear peripheral speed. The measurement equipment used herein would be easy to install in a gearbox and is therefore expected to be applied in actual aeroengine gearboxes.