A rigid tip end section which is connected to an angle section at the fore distal end of an insertion instrument of an ultrasound endoscope is housed in a casing which can be split into a main casing and a separable head block to facilitate maintenance and service of internal component parts of the rigid tip end section. An ultrasound transducer is accommodated in a front side portion of the main casing, while endoscopic observation means including an illumination means and an optical image pickup means are fitted in an inclined wall rising obliquely upward on the rear side of the ultrasound transducer. An outlet opening of a biopsy channel outlet passage is located between the ultrasound transducer and the endoscopic observation means. The main casing is adapted to accommodate the ultrasound transducer and its wiring, while the separable head block is adapted to accommodate at least part of component parts of the endoscopic observation means. The main casing and the separable head block are joined with each other through joint wall portions provided along split lines at the opposite lateral sides and at the front side thereof. A foremost one of angle rings of the angle section is detachably fitted on base end portions of the main casing and separable head block to retain these parts in a fixedly connected state.