To provide a polyester monofilament having sufficient compression properties, a high yarn slippage preventing effect and an excellent impact resistance, free from a trouble of winding around a side yarn and usable as a gut as it is.

The monofilament consists of a polyester having an intrinsic viscosity of ≥0.70, has a linear strength of ≥4.20 cN/dtex and a knot strength of ≥2.20 cN/dtex, has a compressive strength (PT) and a compression deformation rate (PR) in specified ranges, respectively, and has an oval cross section on the measurement of the compressive strength.