A colonoscope as an endoscope has an insertion tube for insertion in a patient's gastrointestinal tract. A head portion is disposed at an end of the insertion tube, for inspecting a large intestine. In combination with this, a device for insertion guide includes a base tube mounted on the head portion. Two propulsion fins are disposed on the base tube to extend in an axial direction of the head portion. A driving mechanism with balloons shifts the propulsion fins between closed and open positions. The propulsion fins, when in the closed position, are disposed to extend along the head portion, and when in the open position, are deployed in a radial direction by setting a distal fin end thereof away from the head portion, to propel the head portion inserted in the large intestine. Furthermore, the balloons set the propulsion fins in the open position alternately with one another.