An underwater mask having an up-down direction and a lateral direction, comprising: a mask body which comprises a lens frame having a lens on a front; and a skirt extending rearward from the lens frame; and a strap configured to be coupled to at least one side of both sides of the mask body through a buckle, the strap having a plurality of engaging teeth, the buckle comprising: a base; a strap adjuster body; a handling member; and a stopper, the base comprising: a second coupling portion configured to be coupled to a first coupling portion on the at least one side turnably in the lateral direction; a fitting hole; and a spring comprising a locking hook, the strap adjuster body comprising: a plurality of engaging teeth opposite to the spring and capable of sequentially engaging with the locking hook; a fitting shaft configured to be fitted into the fitting hole turnably by a predetermined angle in a circumferential direction of the fitting shaft; a guide shaft configured to be wound by the strap; a slide guide groove extending in the up-down direction; and a pair of guide openings opposite to each other in the up-down direction, the handling member comprising: an approximately V-shaped spring; a slider capable of sliding along the slide guide groove; a pair of sliding portion near the slider; and a pair of finger pads outside the pair of sliding portion, the finger pads configured to be exposed from the guide openings of the strap adjuster body, the stopper comprising: a pawl capable of sequentially engaging with the engaging teeth of the strap; and a sliding contact tapered protrusion near the pawl, the sliding contact tapered protrusion slidably disposed between the pair of sliding portion, wherein the stopper is configured to be held by the strap adjuster body swingably toward a direction opposite to the body. a mask body which comprises a lens frame having a lens on a front; and a skirt extending rearward from the lens frame; and a strap configured to be coupled to at least one side of both sides of the mask body through a buckle, the strap having a plurality of engaging teeth, the buckle comprising: a base; a strap adjuster body; a handling member; and a stopper, a second coupling portion configured to be coupled to a first coupling portion on the at least one side turnably in the lateral direction; a fitting hole; and a spring comprising a locking hook, a plurality of engaging teeth opposite to the spring and capable of sequentially engaging with the locking hook; a fitting shaft configured to be fitted into the fitting hole turnably by a predetermined angle in a circumferential direction of the fitting shaft; a guide shaft configured to be wound by the strap; a slide guide groove extending in the up-down direction; and a pair of guide openings opposite to each other in the up-down direction, the handling member comprising: an approximately V-shaped spring; a slider capable of sliding along the slide guide groove; a pair of sliding portion near the slider; and a pair of finger pads outside the pair of sliding portion, the finger pads configured to be exposed from the guide openings of the strap adjuster body, the stopper comprising: a pawl capable of sequentially engaging with the engaging teeth of the strap; and a sliding contact tapered protrusion near the pawl, the sliding contact tapered protrusion slidably disposed between the pair of sliding portion, wherein the stopper is configured to be held by the strap adjuster body swingably toward a direction opposite to the body. an approximately V-shaped spring; a slider capable of sliding along the slide guide groove; a pair of sliding portion near the slider; and a pair of finger pads outside the pair of sliding portion, the finger pads configured to be exposed from the guide openings of the strap adjuster body, the stopper comprising: a pawl capable of sequentially engaging with the engaging teeth of the strap; and a sliding contact tapered protrusion near the pawl, the sliding contact tapered protrusion slidably disposed between the pair of sliding portion, wherein the stopper is configured to be held by the strap adjuster body swingably toward a direction opposite to the body. a pawl capable of sequentially engaging with the engaging teeth of the strap; and a sliding contact tapered protrusion near the pawl, the sliding contact tapered protrusion slidably disposed between the pair of sliding portion, wherein the stopper is configured to be held by the strap adjuster body swingably toward a direction opposite to the body. wherein the stopper is configured to be held by the strap adjuster body swingably toward a direction opposite to the body.