Provided is a protective film which has a polylactic acid-based film or sheet as the substrate. The protective film has such a tear strength as not to break or tear even in manufacturing or processing the protective film or in winding the protective film into a roll. Further, the protective film does not suffer from the melting or deformation of the substrate even at a high temperature exceeding 100 DEG C. The protective film has a removable pressure-sensitive adhesive layer on at least one surface of a substrate, wherein the substrate is composed of a polylactic acid-based film or sheet that contains a polylactic acid (A). The polylactic acid-based film or sheet exhibits a tear strength of 100N/mm or more when torn at least in the machine direction (MD). Further, the dimensional changes of the film or sheet in the machine direction (MD) and the transverse direction (TD) after heating are within the range of +/-3%, while the dimensional change thereof in the machine direction (MD) after heating under load is within the range of +/-3%.