The purpose of this technique is to estimate the lubrication condition in ball bearing with the variation of ultrasonic echo height reflected from a contact surface of an outer ring and a ball. A part of ultrasonic wave sent from a probe attached on a bearing housing is transmitted to the outer ring of ball bearing through a boundary surface of the outer ring and the bearing housing, and arrives at a boundary surface contacting with the ball, and reflects from there. This reflection echo height is used for an evaluation of insufficient supply of oil in ball bearing. When sufficient oil is supplied to an inlet of contact surface between the outer ring and the ball, the echo height which continued decrease as the ball approaches a sound axis of ultrasonic probe is increased locally. However, degree of its increase becomes lower when oil supply is insufficient, and does not appear at all with running out of oil. Also the time lag between bottom of valley in echo height variation curve and the sound axis of ultrasonic probe decreases gradually as supply of oil becomes insufficient, and becomes zero with running out of oil from a contact surface. This evaluation method has a potential which can evaluate the insufficient supply of oil in the ball bearing by two indexes above.