A method for producing a modified polytetrafluoroethylene, the method including: polymerizing tetrafluoroethylene in an aqueous medium in which a polymer having units based on a fluorine-free monomer is present. The fluorine-free monomer is a monomer represented by formula (1): CH2═CR1-L-R2, where R1 is a hydrogen atom or an alkyl group, L is a single bond, —CO—O—*, —O—CO—* or —O—, * is a bonding position to R2, and R2 is an alkyl group. To the total amount of tetrafluoroethylene supplied to the polymerization, a proportion of the polymer is from 0.001 to 0.050 mass %.