To provide a monofilament for a cable-protecting sleeve, being rigid with a high young's modulus and having excellent bending durability and a small filament diameter.

A polyester monofilament for a cable-protecting sleeve contains 25 to 75 wt.% of polyethylenenaphthalate and 25 to 75 wt.% of a polyester other than polyethylenenaphthalate, where the polyester monofilament has a young's modulus of 12000 to 27000 N/mm², a reciprocal folding number of 200 or more until breakage in a bending fatigue test, and a filament diameter variation of 10% or less.