1. A printing method, comprising: detecting a density of a first black image to be printed in a first black color; determining, when the detected density is equal to a first density, a size of a first character to be embedded in the first black image to a first size, and printing the first character at the first size in a second black color with a transmissivity at a near infrared ray band that is different from that of the first black color; determining, when the detected density is lower than the first density, a size of the first character to a second size larger than the first size, and printing the first character at the second size in the second black color; and printing a region except a region in which the first character is to be printed in the first black image, in the first black color.