A protocol converter is configured to be connectable between a field device and a device control apparatus and includes a converter controller and a converter storage. The converter controller acquires field data from the field device, writes the field data to the converter storage as a data set formed by a plurality of pieces of data and changes the value of a first counter controlled by the converter controller when the value of the first counter and the value of a second counter controlled by the device control apparatus are equal, and stops an operation to write the field data to the converter storage when the value of the first counter and the value of the second counter are different.