An ultrasonic endoscope comprising: an insertion section having a distal end and a proximal end; an operation section disposed on the proximal end of the insertion section; an optical observation system disposed in the insertion section, the optical observation system having an objective lens on a distal end surface of the insertion section; an ultrasonic observation system disposed in the insertion section, the ultrasonic observation system having an ultrasonic transducer (52; 52a; 52c) on the distal end surface of the insertion section or further forward than the distal end thereof; and at least two working channels (22, 24) provided in the insertion section, characterized in that the ultrasonic observation system includes: an ultrasonic probe channel having a distal end and disposed in the insertion section; and an ultrasonic probe having a distal end at which the ultrasonic transducer (52; 52a; 52c) is disposed, the ultrasonic probe being inserted in the probe channel so as to be rotatable around its axis, wherein the ultrasonic probe is rotatable when the distal end of the ultrasonic probe is projected further forward than the distal end of the insertion section, and the distal end of the ultrasonic probe is fixable to the distal end of the insertion section when pulled inside the distal end of the insertion section such that a central axis of the ultrasonic transducer (52; 52a; 52c) is directed to at least one of the two working channels (22, 24), and the optical observation system is disposed at the distal end of the insertion section so as to be substantially equidistant from at least the two working channels (22, 24). an insertion section having a distal end and a proximal end; an operation section disposed on the proximal end of the insertion section; an optical observation system disposed in the insertion section, the optical observation system having an objective lens on a distal end surface of the insertion section; an ultrasonic observation system disposed in the insertion section, the ultrasonic observation system having an ultrasonic transducer (52; 52a; 52c) on the distal end surface of the insertion section or further forward than the distal end thereof; and at least two working channels (22, 24) provided in the insertion section, characterized in that the ultrasonic observation system includes: an ultrasonic probe channel having a distal end and disposed in the insertion section; and an ultrasonic probe having a distal end at which the ultrasonic transducer (52; 52a; 52c) is disposed, the ultrasonic probe being inserted in the probe channel so as to be rotatable around its axis, wherein the ultrasonic probe is rotatable when the distal end of the ultrasonic probe is projected further forward than the distal end of the insertion section, and the distal end of the ultrasonic probe is fixable to the distal end of the insertion section when pulled inside the distal end of the insertion section such that a central axis of the ultrasonic transducer (52; 52a; 52c) is directed to at least one of the two working channels (22, 24), and an ultrasonic probe channel having a distal end and disposed in the insertion section; and an ultrasonic probe having a distal end at which the ultrasonic transducer (52; 52a; 52c) is disposed, the ultrasonic probe being inserted in the probe channel so as to be rotatable around its axis, wherein the ultrasonic probe is rotatable when the distal end of the ultrasonic probe is projected further forward than the distal end of the insertion section, and the distal end of the ultrasonic probe is fixable to the distal end of the insertion section when pulled inside the distal end of the insertion section such that a central axis of the ultrasonic transducer (52; 52a; 52c) is directed to at least one of the two working channels (22, 24), and the optical observation system is disposed at the distal end of the insertion section so as to be substantially equidistant from at least the two working channels (22, 24).