A SCADA web HMI system includes a plurality of PLCs, a plurality of HMI client devices, and a SCADA serve device. The SCADA server device includes a communication driver and a client management unit connected to the plurality of HMI client devices. The client management unit transmits in-use screen information indicating a screen currently used by the HMI client devices to the communication driver. The communication driver acquires a screen-specific device list in which parts, and numbers and memory addresses of the PLCs storing the signal data corresponding to the parts are defined, for each of screens displayed on the HMI client devices, selects the signal data defined in the screen-specific device list corresponding to the currently-used screen indicated by the in-use screen information, adds a screen identifier to the selected signal data, and transmits the resultant signal data to the client management unit for each data type.