A protocol converter configured to be connectable between a field device and a device control apparatus, the protocol converter comprising: a converter controller; and a converter storage; wherein the converter storage is configured to have data written thereto and read therefrom by the converter controller and the device control apparatus; and store a first counter controlled by the converter controller and a second counter controlled by the device control apparatus; and wherein the converter controller is configured to acquire field data from the field device; transfer to a first state when the value of the first counter and the value of the second counter are equal; transfer to a second state when the value of the first counter and the value of the second counter are different; in the first state, write the field data to the converter storage as a data set formed by a plurality of pieces of data and change a value of the first counter to a value different from the value of the second counter under the condition that all of the data forming the data set has been written to the converter storage and transfer to the second state; in the second state, stop an operation to write the field data to the converter storage; and in the second state, transfer to the first state when the device control apparatus has changed the value of the second counter to the value equal to the value of the first counter under the condition that the device control apparatus has read all of the data forming the data set from the converter storage; wherein, in the first state, the data set written in the converter storage is discarded when read by the device control apparatus. a converter controller; and a converter storage; wherein the converter storage is configured to have data written thereto and read therefrom by the converter controller and the device control apparatus; and store a first counter controlled by the converter controller and a second counter controlled by the device control apparatus; and have data written thereto and read therefrom by the converter controller and the device control apparatus; and store a first counter controlled by the converter controller and a second counter controlled by the device control apparatus; and wherein the converter controller is configured to acquire field data from the field device; transfer to a first state when the value of the first counter and the value of the second counter are equal; transfer to a second state when the value of the first counter and the value of the second counter are different; in the first state, write the field data to the converter storage as a data set formed by a plurality of pieces of data and change a value of the first counter to a value different from the value of the second counter under the condition that all of the data forming the data set has been written to the converter storage and transfer to the second state; in the second state, stop an operation to write the field data to the converter storage; and in the second state, transfer to the first state when the device control apparatus has changed the value of the second counter to the value equal to the value of the first counter under the condition that the device control apparatus has read all of the data forming the data set from the converter storage; acquire field data from the field device; transfer to a first state when the value of the first counter and the value of the second counter are equal; transfer to a second state when the value of the first counter and the value of the second counter are different; in the first state, write the field data to the converter storage as a data set formed by a plurality of pieces of data and change a value of the first counter to a value different from the value of the second counter under the condition that all of the data forming the data set has been written to the converter storage and transfer to the second state; in the second state, stop an operation to write the field data to the converter storage; and in the second state, transfer to the first state when the device control apparatus has changed the value of the second counter to the value equal to the value of the first counter under the condition that the device control apparatus has read all of the data forming the data set from the converter storage; wherein, in the first state, the data set written in the converter storage is discarded when read by the device control apparatus.