A modified polytetrafluoroethylene comprising a polymer having units based on tetrafluoroethylene, and a polymer having units based on a fluorine-free monomer, wherein the endothermic amount ratio R calculated by the following method is at least 0.65, and wherein the modified polytetrafluoroethylene contains no fluorinated surfactant, Method for calculation of endothermic amount ratio R: Using the modified polytetrafluoroethylene having no history of being heated to a temperature of at least 300°C, measurement is conducted by a differential scanning calorimeter at a heating rate of 20°C/min, to obtain a differential thermal curve; in the obtained differential thermal curve, a temperature lower by 2.5°C from the endothermic peak temperature T p °C located above 340°C is taken as temperature T m °C; and endothermic amount S 1 (mJ/mg) in a temperature range of from 310°C to temperature T m °C and endothermic amount S 2 (mJ/mg) in a temperature range of from temperature T m °C to 360°C are obtained; whereupon the ratio of the endothermic amount S 2 to the endothermic amount S 1 is calculated as the endothermic amount ratio R (endothermic amount S 2 /endothermic amount S 1); for the measurement, the modified polytetrafluoroethylene (10 mg) having no history of being heated at a temperature of at least 300°C is held at a temperature of 200°C for 1 minute and then, heated at the above temperature raising rate to 380°C, to obtain the differential thermal curve showing the temperature on the horizontal axis and the endothermic amount per unit time on the vertical axis; in the obtained differential thermal curve, the temperature T m °C is taken as described above; the endothermic amounts S 1 and S 2 are obtained and R is calculated as described above; for obtaining the endothermic amounts, a baseline is prepared by connecting the point of 310°C and the point of 360°C of the obtained differential thermal curve; the endothermic amount S 1 corresponds to the area of the region of at most temperature T m °C in the region formed from the differential thermal curve and the baseline (area surrounded by the differential thermal curve and the baseline); the endothermic amount S 2 corresponds to the area of the region of at least temperature T m °C, in the region formed by the differential thermal curve and the baseline. the endothermic amount ratio R calculated by the following method is at least 0.65, and wherein the modified polytetrafluoroethylene contains no fluorinated surfactant, Method for calculation of endothermic amount ratio R: Using the modified polytetrafluoroethylene having no history of being heated to a temperature of at least 300°C, measurement is conducted by a differential scanning calorimeter at a heating rate of 20°C/min, to obtain a differential thermal curve; in the obtained differential thermal curve, a temperature lower by 2.5°C from the endothermic peak temperature T p °C located above 340°C is taken as temperature T m °C; and endothermic amount S 1 (mJ/mg) in a temperature range of from 310°C to temperature T m °C and endothermic amount S 2 (mJ/mg) in a temperature range of from temperature T m °C to 360°C are obtained; whereupon the ratio of the endothermic amount S 2 to the endothermic amount S 1 is calculated as the endothermic amount ratio R (endothermic amount S 2 /endothermic amount S 1); for the measurement, the modified polytetrafluoroethylene (10 mg) having no history of being heated at a temperature of at least 300°C is held at a temperature of 200°C for 1 minute and then, heated at the above temperature raising rate to 380°C, to obtain the differential thermal curve showing the temperature on the horizontal axis and the endothermic amount per unit time on the vertical axis; in the obtained differential thermal curve, the temperature T m °C is taken as described above; the endothermic amounts S 1 and S 2 are obtained and R is calculated as described above; for obtaining the endothermic amounts, a baseline is prepared by connecting the point of 310°C and the point of 360°C of the obtained differential thermal curve; the endothermic amount S 1 corresponds to the area of the region of at most temperature T m °C in the region formed from the differential thermal curve and the baseline (area surrounded by the differential thermal curve and the baseline); the endothermic amount S 2 corresponds to the area of the region of at least temperature T m °C, in the region formed by the differential thermal curve and the baseline.