We present a flexible ultrasonic motor that generates linear motion as a novel soft actuator. This motor is comprised of a single cube stator with a through-hole, and a flexible coil spring inserted into the hole. The coil spring, which has a diameter of 10 mm and length of 200 mm, is translated by a vibration generated in the stator. A preload between the stator and the coil spring is important for improving the motor performance. The coil spring inserted into the stator hole generates the preload by expanding in a radial direction. We build a prototype flexible ultrasonic motor, and examine its basic performance parameters such as thrust force, velocity, response time, resolution and flexibility. To demonstrate flexibility, we show the stable thrust force and velocity even when the coil spring is bent to a bending radius of 15 mm.