There is provided a device for monitoring a thickness reduction of an inner surface in a heat transfer tube or an inner surface in an evaporation tube, the device including: a movement unit which moves along a fin tube; a laser measurement unit which is provided in the movement unit and measures the thickness reduction of the inner surface by a laser; a cable which includes a light guiding path for introducing a laser into the laser measurement unit and a light deriving path for transmitting reflected light; and a thickness reduction determining unit which compares the laser measurement data with past data or standard data and determines the current thickness reduction.