An absorbent article for pet configured to be worn in a state of being wrapped around a waist of a pet, and having a longitudinal direction and a width direction, the absorbent article for pet comprising: a liquid permeable top sheet; a liquid impermeable back surface layer; an absorbent core disposed between the top sheet and the back surface layer; a first end portion and a second end portion opposing to each other in the longitudinal direction of the absorbent article for pet; a first side portion and a second side portion opposing to each other in the width direction of the absorbent article for pet; a first gather portion that is disposed on a top sheet side of the first side portion and configured to stand upright from the top sheet; a second gather portion that is disposed on the top sheet side of the second side portion and configured to stand upright from the top sheet; and a pair of releasably-joined portions in which the first gather portion is releasably-joined to itself in a state of being folded outward in the width direction of the absorbent article for pet, in the first end portion and the second end portion, wherein, the first gather portion includes: a first side sheet that is disposed on the top sheet side of the first side portion, has an outer edge joined with the top sheet or the back surface layer and has an inner edge at least a part of which is a free end; and an elastic member that is attached to an inner edge side of the first side sheet in an extended state; the second gather portion includes: a second side sheet that is disposed on the top sheet side of the second side portion, has an outer edge joined with the top sheet or the back surface layer and has an inner edge at least a part of which is a free end; and an elastic member that is attached to an inner edge side of the second side sheet in an extended state; and the pair of releasably-joined portions is defined by the first side sheet, and wherein a density of the first side sheet is greater than a density of the top sheet. a liquid permeable top sheet; a liquid impermeable back surface layer; an absorbent core disposed between the top sheet and the back surface layer; a first end portion and a second end portion opposing to each other in the longitudinal direction of the absorbent article for pet; a first side portion and a second side portion opposing to each other in the width direction of the absorbent article for pet; a first gather portion that is disposed on a top sheet side of the first side portion and configured to stand upright from the top sheet; a second gather portion that is disposed on the top sheet side of the second side portion and configured to stand upright from the top sheet; and a pair of releasably-joined portions in which the first gather portion is releasably-joined to itself in a state of being folded outward in the width direction of the absorbent article for pet, in the first end portion and the second end portion, wherein, the first gather portion includes: a first side sheet that is disposed on the top sheet side of the first side portion, has an outer edge joined with the top sheet or the back surface layer and has an inner edge at least a part of which is a free end; and an elastic member that is attached to an inner edge side of the first side sheet in an extended state; a first side sheet that is disposed on the top sheet side of the first side portion, has an outer edge joined with the top sheet or the back surface layer and has an inner edge at least a part of which is a free end; and an elastic member that is attached to an inner edge side of the first side sheet in an extended state; the second gather portion includes: a second side sheet that is disposed on the top sheet side of the second side portion, has an outer edge joined with the top sheet or the back surface layer and has an inner edge at least a part of which is a free end; and an elastic member that is attached to an inner edge side of the second side sheet in an extended state; and a second side sheet that is disposed on the top sheet side of the second side portion, has an outer edge joined with the top sheet or the back surface layer and has an inner edge at least a part of which is a free end; and an elastic member that is attached to an inner edge side of the second side sheet in an extended state; and the pair of releasably-joined portions is defined by the first side sheet, and wherein a density of the first side sheet is greater than a density of the top sheet.