A sealing device for an endoscope intended to effect sealing between an endoscope guide hole and an operating wire inserted thereinto is set in a medical instrument-leading chamber provided in a distal end portion of an endoscope, and is formed of a substantially hollow cylindrical or trumpet-shaped elastic member. One end of the sealing device is hermetically connected to a wall of the medical instrument-leading chamber to which the endoscope guide hole in the distal end portion is opened. The other end of the sealing device is also hermetically connected to the intermediate point of that portion of the operating wire which projects into the medical instrument-leading chamber from the endoscope guide hole, the operating wire being used to turn a rotary member in order to define the direction in which the forward portion of the medical instrument is to be directed in a body cavity. This arrangement effectively prevents a coelom liquid, washing water or liquid filth from being carried into the endoscope guide hole.