a plurality of Ising machines that each search for a solution of a subproblem among a plurality of subproblems obtained by dividing a problem represented by an Ising model in a pipeline manner,

each of the plurality of Ising machines including:

a memory configured to store values of a state variable set corresponding to a subproblem to be calculated for each group of a plurality of groups;

a processor coupled to the memory and the processor configured to:

perform processing of calculating a first solution candidate for each group of the plurality of groups by a corresponding Ising machine of the plurality of Ising machines;

changing each group processed by each Ising machine of the plurality of Ising machines to another group of the plurality of groups to be processed every unit period;

perform processing of calculating a second solution candidate for each group of the plurality of groups by each Ising machines of the plurality of Ising machines based on the first solution candidates of subproblems calculated by other Ising machines such that the second solution candidate is calculated for the state variable set of the another group different from groups processed by the other Ising machines in a same unit period, such that processing is executed in parallel;

and

a communication circuit configured to transmit the second solution candidate to the other Ising machines every time the unit period ends.