The present disclosure provides an automatic belt tensioner for an engine which automatically adjusts the tension of a belt for transmitting rotational force between an engine and an auxiliary machinery component. The automatic belt tensioner includes: a tensioner body mounted at the engine side and having an internal space; a tension spring installed in the internal space of the tensioner body; first and second damping shoes coupled by the tension spring; a first arm having a hinge portion coupled to the first damping shoe; a second arm having a hinge portion coupled to the second damping shoe; and idlers rotatably mounted on the first and second arms, respectively, and also supporting the belt. The first hinge portion which is a rotation center of the first arm and the second hinge portion which is a rotation center of the second arm are rotatably coupled to the tensioner body.