An ethylene/tetrafluoroethylene copolymer satisfying the following formula, wherein peak intensities, determined by Fourier transform infrared spectroscopy, of vibrations derived from a -CF 2 H group, a -CF 2 CH 2 COF group, a -COF group, a -COOH group, a dimer of a -CF 2 COOH group and a monomer of a CF 2 CH 2 COOH group, a -COOCH 3 group, a -CONH 2 group, and a - CH 2 OH group satisfy the following formula, 75 ≤ tanδ 60 / tanδ 5 × 100 ≤ 225 wherein tanδ: a loss tangent after five minutes from start of measurement in dynamic mechanical analysis in an air atmosphere at 320°C; and tanδ: a loss tangent after 60 minutes from the start of measurement in the dynamic mechanical analysis in the air atmosphere at 320°C, whereby tanδ and tanδ determined as disclosed in the description, PI A / PI B + PI C + PI D + PI E + PI F + PI G + PI H ≥ 0.60 wherein PI A: the peak intensity of vibration derived from the -CF 2 H group; PI B: the peak intensity of vibration derived from the -CF 2 CH 2 COF group; PI C: the peak intensity of vibration derived from the -COF group; PI D: the peak intensity of vibration derived from the -COOH group; PI E: the peak intensity of vibration derived from the dimer of the -CF 2 COOH group and the monomer of the - CF 2 CH 2 COOH group; PI F: the peak intensity of vibration derived from the -COOCH 3 group; PI G: the peak intensity of vibration derived from the -CONH 2 group; and PI H: the peak intensity of vibration derived from the -CH 2 OH group, whereby PI A, PI B, PI C, PI D, PI E, PI F, PI G and PI H are determined in accordance with the method disclosed in the description. tanδ: a loss tangent after five minutes from start of measurement in dynamic mechanical analysis in an air atmosphere at 320°C; and tanδ: a loss tangent after 60 minutes from the start of measurement in the dynamic mechanical analysis in the air atmosphere at 320°C, whereby tanδ and tanδ determined as disclosed in the description, PI A / PI B + PI C + PI D + PI E + PI F + PI G + PI H ≥ 0.60 wherein PI A: the peak intensity of vibration derived from the -CF 2 H group; PI B: the peak intensity of vibration derived from the -CF 2 CH 2 COF group; PI C: the peak intensity of vibration derived from the -COF group; PI D: the peak intensity of vibration derived from the -COOH group; PI E: the peak intensity of vibration derived from the dimer of the -CF 2 COOH group and the monomer of the - CF 2 CH 2 COOH group; PI F: the peak intensity of vibration derived from the -COOCH 3 group; PI G: the peak intensity of vibration derived from the -CONH 2 group; and PI H: the peak intensity of vibration derived from the -CH 2 OH group, whereby PI A, PI B, PI C, PI D, PI E, PI F, PI G and PI H are determined in accordance with the method disclosed in the description. PI A: the peak intensity of vibration derived from the -CF 2 H group; PI B: the peak intensity of vibration derived from the -CF 2 CH 2 COF group; PI C: the peak intensity of vibration derived from the -COF group; PI D: the peak intensity of vibration derived from the -COOH group; PI E: the peak intensity of vibration derived from the dimer of the -CF 2 COOH group and the monomer of the - CF 2 CH 2 COOH group; PI F: the peak intensity of vibration derived from the -COOCH 3 group; PI G: the peak intensity of vibration derived from the -CONH 2 group; and PI H: the peak intensity of vibration derived from the -CH 2 OH group, whereby PI A, PI B, PI C, PI D, PI E, PI F, PI G and PI H are determined in accordance with the method disclosed in the description.