An ink contains at least a first solid particle, and a second solid particle formed of a base material of a different main component from that of the first solid particle. The first solid particle and the second solid particle have zeta potentials of the same polarity, or zeta potentials of 0±5 mV. The first and second solid particles in the ink have the same surface property, specifically the same interface property in the ink. This makes it possible to use a common dispersant suited for adsorption on the first and second solid particles. In this way, more than one kind of solid particle can be stably dispersed using a sole kind of dispersant.