An ink set comprising: an ink composition which contains a colorant and water; and a treatment liquid which contains water, resin particles which are water-insoluble such that the resin particles can be detected and the volume average particle diameter measured using a 1% to 2% by mass aqueous dispersion of the resin particles at 25°C, and in which the content of a carboxy group or a salt of the carboxy group is in the range of 1.0 mmol to 7.0 mmol per 1 g of the water-insoluble resin particles, and a compound that causes the colorant in the ink composition to aggregate which is an inorganic acidic compound, an organic acidic compound or a combination thereof, the organic acidic compound being one or more of malonic acid, malic acid, maleic acid, succinic acid, glutaric acid, fumaric acid, tartaric acid, 4-methylphthalic acid and citric acid; wherein the water-insoluble resin has constitutional units derived from a monomer containing a sulfo group or a salt of the sulfo group, and constitutional units represented by Formula or Formula wherein the total content of constitutional units represented by Formula and Formula is in the range of 10% by mass to 50% by mass with respect to the total mass of the water-insoluble resin particles: wherein R 1 represents a methyl group or a hydrogen atom; L 1 represents a single bond, a linear, branched or cyclic alkylene group having 1 to 12 carbon atoms, an arylene group having 6 to 12 carbon atoms, -O-, -NH-, -S-, -C(=O)- or a divalent linking group formed by two or more of these being linked to each other; M represents a hydrogen atom or a cation; and L 2 and L 3 each independently represent a single bond or a methylene group. an ink composition which contains a colorant and water; and a treatment liquid which contains water, resin particles which are water-insoluble such that the resin particles can be detected and the volume average particle diameter measured using a 1% to 2% by mass aqueous dispersion of the resin particles at 25°C, and in which the content of a carboxy group or a salt of the carboxy group is in the range of 1.0 mmol to 7.0 mmol per 1 g of the water-insoluble resin particles, and a compound that causes the colorant in the ink composition to aggregate which is an inorganic acidic compound, an organic acidic compound or a combination thereof, the organic acidic compound being one or more of malonic acid, malic acid, maleic acid, succinic acid, glutaric acid, fumaric acid, tartaric acid, 4-methylphthalic acid and citric acid; wherein the water-insoluble resin has constitutional units derived from a monomer containing a sulfo group or a salt of the sulfo group, and constitutional units represented by Formula or Formula wherein the total content of constitutional units represented by Formula and Formula is in the range of 10% by mass to 50% by mass with respect to the total mass of the water-insoluble resin particles: wherein R 1 represents a methyl group or a hydrogen atom; L 1 represents a single bond, a linear, branched or cyclic alkylene group having 1 to 12 carbon atoms, an arylene group having 6 to 12 carbon atoms, -O-, -NH-, -S-, -C(=O)- or a divalent linking group formed by two or more of these being linked to each other; M represents a hydrogen atom or a cation; and L 2 and L 3 each independently represent a single bond or a methylene group.