An ultrasonic diagnostic apparatus including an ultrasonic vibrating element, a rotator for rotating the ultrasonic vibrating element, an illuminating window and an observing window arranged in a side wall of an inserting portion. The apparatus is so arranged that the center axis of the inserting portion is deviated from that of the rotator with respect to the radial direction but the center of curvature of a cover window surrounding the rotator is coincident with the center axis of the rotator. Therefore, a uniform resolving power can be obtained in all directions into which the ultrasonic wave is transmitted. Then an ultrasonic image without undesired artificates is displayed on a screen of a monitor and the outer diameter of the inserting portion can be made small.