1. An information processing unit calculating a ground state which is a spin alignment with minimum energy of an original problem that is an Ising model, or an approximate solution of the ground state as a solution of the original problem, the information processing unit comprising: a sub-problem generation unit that generates one or more sub-problems, which are Ising models, from the original problem; a ground-state search unit that searches for a ground state of a problem of an Ising model having specified multiple types of coefficient values and searches for a ground state of each of the sub-problems generated by the sub-problem generation unit; and a solution generation unit that generates the solution of the original problem based on a solution of each sub-problem obtained by the search made by the ground-state search unit; wherein the sub-problem generation unit generates the sub-problems which are the Ising models having one or more coefficient values selected from the multiple types based on one or more coefficient values of the Ising model of the original problem.