1. A problem solver for generating an action sequence executed from an initial state to a goal state in a state space defined by a particular problem, comprising: a cognitive distance learning unit learning a cognitive distance that represents a cost acted on the environment of the state space, the cost being spent in an action sequence executed from a first state in the state space to a second state that is different from the first state; and a next action deciding unit deciding a next action contained in the action sequence that has to be executed in a particular state to attain the goal state in the state space based on the cognitive distance learnt by said cognitive distance learning unit.