A plurality of heating wires 2 have a plurality of detours 1 formed therein, the heating wires 2 have inlets 10 formed therein for connection with the detours 1, and both ends of each of the inlets 10 and both ends of each of the detours 1 are connected by a pair of connection lines 9. When a shorter one of the shortest spacing between the paired connection lines 9 and the shortest spacing between both ends of the inlets 10 is called We, when the maximum width of the detours 1 in a horizontal direction is called Wr, when a received frequency has a wavelength of λ0 in the air, and when the formula of {(Wr−We)/2}=L1 is established, the formula of L1≧0.0053·λ0 is satisfied.