To provide a belt protecting device for an engine capable of preventing biting of foreign matter while suppressing increase in cost.

This belt protecting device 40 protects an auxiliary machine drive belt 15 wound around and stretched between auxiliary machine pulleys 16, 17 and a crank pulley 13 of an engine 11. The auxiliary machine drive belt 15 is wound and stretched from below between the crank pulley 13 and the first auxiliary machine pulley 16 at a rear side by one relative to the crank pulley 13 in the circulating direction of the auxiliary machine drive belt 15. In the belt protecting device 40, a belt tensioner 21 applying tension to the auxiliary machine drive belt 15 is arranged between the crank pulley 13 and the first auxiliary machine pulleys 16. A portion of the belt tensioner 21 is at least arranged at a region in proximity with a winding start position P of the auxiliary machine drive belt 15 relative to the crank pulley 13 to establish a foreign matter biting regulation part 38.