In an endoscope apparatus of the present invention, an one-side gap between a tube main body of an overtube and an insertion portion is set at 0.5 mm to 1.5 mm. Lubricity is improved because a lubricating liquid such as water supplied from the injection port to the base end side of the tube main body is sufficiently supplied to the whole area of the gap between the tube main body and the insertion portion by setting the one-side gap at not less than 0.5 mm. Also, the snaking of the insertion portion with respect to the tube main body can be held to a minimum by setting the one-side gap at not more than 1.5 mm. As a result of this, the insertion and extraction operability of the insertion portion with respect to the overtube is improved.