A novel water-based latex ink has been developed, which is capable of forming images with ink-jet printers on non-osmotic media such as plastic media. This water-based latex ink provides image quality and discharge reliability comparable with those of solvent inks applicable to non-osmotic media that have been widely used in the field of sign graphics. This water-based latex ink also provides images with safety and high quality on osmotic media that are used for wallpaper, etc. We have discovered that, for improving image quality with water-based latex inks, it is important to suppress aggregation of particles when the ink undergoes a drying process, which has a great relation to the SP value (i.e., Solubility Parameter) of solvents. We have also discovered that water-based latex inks form skin layer as water evaporates at the meniscus part of ink-discharge nozzle, decreasing discharge reliability.