An ultrasonic endoscope apparatus in which an ultrasonic vibrating element radiating an ultrasonic wave to obtain an image of an object to be inspected is rotatably housed in a distal end of an insertion section of the endoscope, and a rotation angle sensing mechanism is also housed in the distal end. The rotation angle sensing mechanism has a magnetic recording section having a magnetization pattern for forming a magnetic field, and a magnetic sensor having at least one magneto-resistive element for sensing the magnetic field. One of the magnetic recording section and the magnetic sensor is rotated integrally with the ultrasonic vibrating element so that a rotation angle of the ultrasonic vibrating element is sensed in a precise manner.