To provide a field communication system which improves the precision of a control operation by detecting the operation execution cycle of each device through communication and changing the operation cycle of each device.

An operation controlling part 20 operates process data which are inputted from a device controlling part 6 and a device data inputting part 19, sets data for communication and time cycle output data and stores the in a data outputting part 21 and a time cycle outputting part 22. A data transmitting and receiving part 13 sends data of the parts 21 and 22 to a field bus 4 through a network controlling part 12. A controlling part 12 of other blocks transfers data for self-station to the part 13 from the bus 4, takes out time cycle data and outputs it together with other data to a time cycle inputting part 14. The part 14 decides a time cycle updation cycle and stores it in a preservation memory part 15, and an operation execution controlling part 18 controls the data supply of the part 15 to an operation controlling part 20. It inputs data of the part 15 in the part 20 through a preservation data selecting part 16 and carries out a control operation.