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.