A method and system of specifying a device that is the root cause of impeding productivity of a production line in consideration of even mutual influence among processes of production fluctuation with respect to the subject of specifying a device in which productive capacity is reduced due to a problem that the productive capacity of the device is changed due to production that one machine works for multiple process and a problem that the productive capacity of device is changed due to high product mix and low product volume production. A measure for changing productive capacity of devices intentionally and simulating influence to the whole production system, a measure for measuring mutual influence among processes of production fluctuation produced by the simulation and a measure for specifying a device that is the root cause of impeding the productivity on the basis of the measured result are provided.