The present invention relates to a door closer, in a housing of which is received a sliding cylinder, the sliding cylinder is provided with teeth on an outer periphery thereof for meshing with a gear wheel of a door shaft, a sliding cylinder interiorly formed with inner wall serves to move relative to a piston of a piston shaft. At an end of the piston shaft is provided with an oil-replenishing cylinder that serves to replenish hydraulic oil to the sliding cylinder via a one-way valve.