PURPOSE: To provide a device whereby erosion wearing in a blade of a gas turbine, for instance, used for blast furnace top pressure power generation, can be easily and inexpensively detected.

CONSTITUTION: A hollow vessel 1, constituted of metal equivalent to a quality and/or hardness of or worn easier than a blade of a gas turbine, is filled with insulating powder 2, and in this vessel, a pair of lead wires 3, 3' are arranged with a predetermined space to constitute a sensor mounted to a gas flow path of the gas turbine. Voltage is applied to the lead wires 3, 3' by a DC power supply 4 to connect a relay 6 to this circuit and further to provide an alarm device 7 for generating an alarm by actuating the relay 6. In this way, when an outer shell of the hollow vessel 1 is broken by wearing due to impurities such as dust or the like, the insulating powder 2 is scattered to bring the two lead wires 3, 3' into contact, and by flowing a current, the relay 6 is actuated to generate the alarm from the alarm device 7.