A men's undergarment comprises an upper portion (2) looped to encircle the waist of a wearer; a front portion supported by the upper portion (2) and extending downwards from it; a rear area (5), extending downwards from the upper portion (2) and connected to the front portion, with which it forms a first and a second opening (6, 7) for the legs of the wearer; the rear area comprising a portion (5a, 5b, 5c) made of elastic fabric and comprising a system (9) for confining the elastic stretching of the rear portion (5a, 5b, 5c); the elastic stretching confining system comprising at least one sector (14) in the rear area (5) where stretching is reduced compared with the stretching of the remaining part of the rear area (5).