A fieldbus adapter connected between a fieldbus conforming to a foundation fieldbus, FF, standard specification and handling a digital signal and a field device handling an analog signal, the fieldbus adapter comprising: a first connection unit detachably connected to the fieldbus; a second connection unit detachably connected to the field device; a protocol stack conforming to one of FF H1 bus and HSE bus corresponding to the fieldbus; a virtual field device object conforming to the foundation fieldbus standard specification, the virtual field device object including a function block application that includes an analog input block and an analog output block that are function blocks conforming to the foundation fieldbus standard specification; and an analog/digital conversion unit provided between the first connection unit and the second connection unit, the analog/digital conversion unit being configured to bidirectionally convert the digital signal handled by the fieldbus and the analog signal handled by the field device, wherein the analog/digital conversion unit is configured to read a primary value of the field device, which is an analog signal of 4 to 20 mA, and to convert the primary value into a digital value, the analog input block is configured to receive the digital value, and to output digital data that is a calculation result in the analog input block to the fieldbus via the protocol stack, the analog output block is configured to receive the digital data output from another field device or a manipulation monitoring apparatus to the fieldbus via the protocol stack, the analog/digital conversion unit is configured to convert a calculation result in the analog output block into an analog value and to output the analog value to the field device, and the field device is configured to receive the analog value. a first connection unit detachably connected to the fieldbus; a second connection unit detachably connected to the field device; a protocol stack conforming to one of FF H1 bus and HSE bus corresponding to the fieldbus; a virtual field device object conforming to the foundation fieldbus standard specification, the virtual field device object including a function block application that includes an analog input block and an analog output block that are function blocks conforming to the foundation fieldbus standard specification; and an analog/digital conversion unit provided between the first connection unit and the second connection unit, the analog/digital conversion unit being configured to bidirectionally convert the digital signal handled by the fieldbus and the analog signal handled by the field device, wherein the analog/digital conversion unit is configured to read a primary value of the field device, which is an analog signal of 4 to 20 mA, and to convert the primary value into a digital value, the analog input block is configured to receive the digital value, and to output digital data that is a calculation result in the analog input block to the fieldbus via the protocol stack, the analog output block is configured to receive the digital data output from another field device or a manipulation monitoring apparatus to the fieldbus via the protocol stack, the analog/digital conversion unit is configured to convert a calculation result in the analog output block into an analog value and to output the analog value to the field device, and the field device is configured to receive the analog value.