A metallic reflection plate is disposed so as to face oppositely an antenna connected to a transmitter-receiver. A detection wave from the antenna is reflected by the reflection plate so as to enter an interior of a blast furnace through an opening portion of the furnace and the detection wave reflected by a substance loaded in the furnace is reflected by the reflection plate and sent to the antenna. In a main pipe portion accommodating the antenna and the reflection plate, a gas-permeable bulkhead is provided so as to face oppositely a reflection surface of the reflection plate and a non-gas-permeable bulkhead is provided so as to face oppositely an antenna surface of the antenna. An inactive gas is supplied under high pressure into a space defined between the gas-permeable bulkhead and the non-gas-permeable bulkhead.
