An ultrasound endoscope having an ultrasound transducer mounted on a rigid tip end section at the distal end of an elongated endoscopic insert section, on the front side of a inclined casing wall section in which illumination windows and an optical image pickup assembly are fitted. For protrusion of a medical instrument into a body cavity, an instrument outlet of a biopsy channel is opened in a casing of the rigid tip end section in an obliquely upward direction from behind the ultrasound transducer. As far as a proximal end of the rigid tip end section, the biopsy channel is constituted by a flexible tube which is extended in the axial direction of the insert section and joined by way of a curved connecting pipe with a terminal passage which is formed internally of a casing of the rigid tip end section and inclined relative to the longitudinal axis of the latter. The instrument outlet is opened in a flat top surface of a plateau which is provided on the casing of the rigid tip end section on the rear side of the ultrasound transducer, the flat top surface of said plateau being located at a level lower than a top end the ultrasound transducer and the illumination windows fitted in the inclined casing wall section.