Among high-speed On/Off valves used for PWM control of oil hydraulic pressure, a 3-way ball valve has the advantage of structural simplicity. To improve the performance of a solenoid-driven ball valve, one of the requirements is to reduce so-called 'fluid force' working on the ball. The present paper aims to clarify how such fluid force is produced in a 3-way ball valve with a steady-state oil flow. Measurements of pressure losses, flow rates, ball thrusts and displacements, in addition to computational studies on the flow around the ball, have revealed the following : the pressure falls along the two constriction gaps formed between the ball and valve seats on the supply and drain sides substantially dominate the fluid force. The gap on the drain side works to increase the thrust on the ball, while the one on the supply side decreases it. Furthermore, the structural modification of the valve seat based on the present findings has proved to effectively reduce the fluid force on the ball.