A test pattern generator includes a random command address generator suitable for generating N combinations, each combination of a command and an address, where N is an integer greater than or equal to 2; an address converter suitable for converting the N combinations into an N-dimensional address; a history storage circuit which is accessed based on the N-dimensional address; and a controller suitable for classifying the N combinations as issue targets, when an area in the history storage circuit, which is accessed based on the N-dimensional address, indicates a value of no hit.