Ink set and duplex printing method

A duplex printing method of performing duplex printing by using an ink jet recording apparatus, wherein

• the ink jet recording apparatus includes an ink set and a sheet transporting mechanism,
• the ink set contains at least one selected from a group of a black pigment ink, a self-dispersing color pigment ink, and a resin-dispersion color pigment ink,
• a moisture quantity of the black pigment ink is from 68 mass % to 75 mass %,
• a moisture quantity of the self-dispersing color pigment ink is from 62 mass % to 72 mass %,
• a moisture quantity of the resin-dispersion color pigment ink is from 68 mass % to 75 mass %,
• each of the black pigment ink, the self-dispersing color pigment ink, and the resin-dispersion color pigment ink further contains an alkylene glycol monoalkyl ether in an amount that ranges between 0.5 mass % to 6 mass %,
• each of the black pigment ink, the self-dispersing color pigment ink, and the resin-dispersion color pigment ink further contains trimethylglycine,
• each of the black pigment ink, the self-dispersing color pigment ink, and the resin-dispersion color pigment ink contains three or more types of acetylene glycol-based surfactants selected from a group of acetylene glycol and an ethylene oxide adduct of acetylene glycol, and the total content of the acetylene glycol-based surfactants is from 0.1 mass % to 3 mass % with respect to the total mass of each of the black pigment ink, the self-dispersing color pigment ink, and the resin-dispersion color pigment ink, and
• in the sheet transporting mechanism, a period from when printing on a front surface is started, until printing on a back surface is started is within three seconds.