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. <EOS>