In a mass flow control system which comprises a first apparatus that is a mass flow controller, an external sensor that is at least one detection means constituting a second apparatus that is an apparatus disposed outside said first apparatus and at least one control section prepared in either one or both of housings of said first apparatus and said second apparatus, and is configured so as to control a flow rate of fluid flowing through a channel, the control section is configured such that opening of a flow control valve can be controlled based on at least an external signal that is a detection signal output from the external sensor.