1. a quantization method, comprising the steps of: quantizing a first image data by use of a basic pattern; and converting the first image data into a second image data that represents a binary or multi-level quantized pattern having a gray level smaller than that of the first image data, wherein, when each of basic tone frequencies is a local maximum of spatial frequency components in a pattern image that contains each of basic tone patterns repeatedly arranged as a repeating unit of a specific pattern in a two-dimensional dot arrangement, the basic pattern has spatial frequency characteristics in which components at and in a neighborhood of each basic tone frequency are relatively suppressed in comparison with other spatial frequency components in the pattern image, the local maximum exists in components at an outer periphery of each basic tone frequency outside the neighborhood of each basic tone frequency, and components at low frequencies are suppressed.