The invention includes: an ultrasound transducer a holding part that holds the ultrasound transducer an insertion portion in a rigid cylindrical shape where the holding part is installed at a distal end and an insertion assisting instrument in a rigid rod or cylindrical shape-disposed in an extractable manner in the insertion portion wherein: the holding part is installed at the distal end of the insertion portion to form a shape that is curved with a predetermined angle in a separating direction from a longitudinal direction from the distal end of the insertion portion and a curvature angle of the holding part to the longitudinal direction of the insertion portion is set so that, when the insertion assisting instrument is inserted in the insertion portion, a straight line that contacts a distal end face of the insertion assisting instrument contacts the ultrasound transmitting/receiving face.
