A surface detection device of a loaded material in a blast furnace , which comprises a reflecting plate installed just above an opening provided in the vicinity of the top of the blast furnace and having a reflecting surface inclined toward the opening, an antenna installed to face the reflecting surface of the reflecting plate , and a waveguide configured to couple the antenna and a detection wave transmitting/receiving unit , wherein the device is configured to transmit a detection wave from the antenna to the reflecting surface of the reflecting plate , to enable the detection wave to enter the furnace through the opening , to enable the detection wave reflected from the loaded material in the furnace to return to the reflecting surface of the reflecting plate through the opening , to send the same to the antenna , to detect the detection wave by the detection wave transmitting/receiving unit and to detect a distance to a surface of the loaded material or a surface profile of the loaded material, said device comprising: a coupler configured to couple the waveguide and the transmitting/receiving unit at a state where the waveguide and the transmitting/receiving unit are separated, a waveguide rotating unit configured to rotate the waveguide about an axis line of the waveguide by a predetermined angle, the waveguide rotating unit comprising a motor , a motor-side gear and a waveguide-side gear , the motor-side gear being configured to be rotated by the motor and the waveguide-side gear being attached to the waveguide , the motor-side gear and the waveguide-side gear being configured to transmit rotation from the motor to the waveguide , wherein the antenna and the reflecting plate are coupled by a coupling member , wherein the coupling member is a cylinder-shaped member, wherein one end of the coupling member is fixed to an opening periphery edge of the antenna , wherein the reflecting plate is fixed to the other end of the coupling member , or two support members extend from the other end of the coupling member and support shafts provided at both diametrical ends of the reflecting plate , and the reflecting plate is configured to rotate about the axis line of the waveguide together with rotation of the antenna by the waveguide rotating unit with the transmitting/receiving unit being stationary. a coupler configured to couple the waveguide and the transmitting/receiving unit at a state where the waveguide and the transmitting/receiving unit are separated, a waveguide rotating unit configured to rotate the waveguide about an axis line of the waveguide by a predetermined angle, the waveguide rotating unit comprising a motor , a motor-side gear and a waveguide-side gear , the motor-side gear being configured to be rotated by the motor and the waveguide-side gear being attached to the waveguide , the motor-side gear and the waveguide-side gear being configured to transmit rotation from the motor to the waveguide , wherein the antenna and the reflecting plate are coupled by a coupling member , wherein the coupling member is a cylinder-shaped member, wherein one end of the coupling member is fixed to an opening periphery edge of the antenna , wherein the reflecting plate is fixed to the other end of the coupling member , or two support members extend from the other end of the coupling member and support shafts provided at both diametrical ends of the reflecting plate , and the reflecting plate is configured to rotate about the axis line of the waveguide together with rotation of the antenna by the waveguide rotating unit with the transmitting/receiving unit being stationary. <EOS>