A monitor system comprising: a programmable controller for executing a user program stored in a user program memory of said programmable controller to carry out a control, a data collecting unit connected to said programmable controller for collecting real variable data of said programmable controller; a monitor device connected to said data collecting unit through a communication line; displaying means and a display device; wherein said data collecting unit comprises: memory means for storing a preliminarily defined virtual variable, real variables of said programmable controller, and a calculation formula using data from one or more real variables for obtaining virtual variable data of said virtual variable; collecting means for collecting the real variable data of the one or more real variables to be used in said calculation formula and any other real variable data from other real variables of said programmable controller; and calculating means for carrying out a calculation according to the calculation formula based on the real variable data collected by said collecting means; and wherein said memory means includes means for creating mapping data, said mapping data consisting of the addresses in a memory of said programmable controller of the one or more real variables used in said stored calculation formula and the addresses of said any other real variables corresponding to said any other real variable data to be collected by said collecting means; wherein said collecting means includes means for collecting said real variable data from a memory of said programmable controller based on said mapping data in synchronism with the cycle time of said programmable controller; wherein said calculating means includes means for generating a data array of virtual variable data by substituting the one or more real variables used in said stored calculation formula by the corresponding real variable data obtained by said collecting means, and by carrying out a calculation according to said stored formula; wherein said monitor device comprises setting-registering means for setting and registering the virtual variable and the calculation formula stored in said memory means through a setting screen and means for transmitting the setting data inclusive of the calculation formula to the data collecting unit; and wherein said displaying means are adapted to display correspondence of the real variable data collected by said collecting means and the result of the calculation by said calculation means as waveforms on said display device. a programmable controller for executing a user program stored in a user program memory of said programmable controller to carry out a control, a data collecting unit connected to said programmable controller for collecting real variable data of said programmable controller; a monitor device connected to said data collecting unit through a communication line; displaying means and a display device; memory means for storing a preliminarily defined virtual variable, real variables of said programmable controller, and a calculation formula using data from one or more real variables for obtaining virtual variable data of said virtual variable; collecting means for collecting the real variable data of the one or more real variables to be used in said calculation formula and any other real variable data from other real variables of said programmable controller; and calculating means for carrying out a calculation according to the calculation formula based on the real variable data collected by said collecting means; and wherein said memory means includes means for creating mapping data, said mapping data consisting of the addresses in a memory of said programmable controller of the one or more real variables used in said stored calculation formula and the addresses of said any other real variables corresponding to said any other real variable data to be collected by said collecting means; wherein said collecting means includes means for collecting said real variable data from a memory of said programmable controller based on said mapping data in synchronism with the cycle time of said programmable controller; wherein said calculating means includes means for generating a data array of virtual variable data by substituting the one or more real variables used in said stored calculation formula by the corresponding real variable data obtained by said collecting means, and by carrying out a calculation according to said stored formula; wherein said monitor device comprises setting-registering means for setting and registering the virtual variable and the calculation formula stored in said memory means through a setting screen and means for transmitting the setting data inclusive of the calculation formula to the data collecting unit; and wherein said displaying means are adapted to display correspondence of the real variable data collected by said collecting means and the result of the calculation by said calculation means as waveforms on said display device.