A calibration method includes: detecting a predetermined position of a work machine according to first and second methods in a different posture of the work machine; and obtaining a conversion information item used to convert a position detected by the first method from a coordinate system in the first method into a coordinate system different from that of the first method or obtaining a conversion information item used to convert a position detected by the second method from a coordinate system of the second method into a coordinate system different from that of the second method by using a first position information item as information for the predetermined position detected by the first method and a second position information item as information for the predetermined position detected by the second method in a posture of the work machine when the predetermined position is detected by the first method.