To provide an electrothermal mat capable of preventing adjacent electrothermal wires from shorting even if kink is generated, and winding up the electrothermal wires at a suitable interval according to wire diameters.

This electrothermal mat for molding an electric welding joint spirally winds electrothermal wires 14 around a through hole provided on the center portion of a synthetic resin sheet. In this electrothermal mat, assuming that the wire expansion rate of the electrothermal wire is α, the diameter of the electrothermal wire is d, the diameter of the most outer peripheral portion of the electrothermal wire spirally wound is D, a temperature rise until the electrothermal wire stars buckling is ΔT, and a temperature deducting the ΔT from a temperature with which resin of a joining part is melted by heating of the electrothermal wire is ΔT', a value H is obtained from the below expression (1). H=/2[(d/4√αΔT+DαΔT)2(1+αΔT)2-(d/4√αΔT-DαΔT')2]1/2...(1) The interval W of the electrothermal wire in the most outer peripheral portion is set in a range indicated by the below expression (2). 2H>W>H...(2).