A maintenance managing apparatus comprising: a device information acquirer configured to acquire device information including a change in a state of a field device; an estimation rule storage which stores an estimation rule associating each of multiple changes in the state of the field device with a respective maintenance operation performed to the field device; a maintenance operation estimator configured to: determine whether or not the change in the state of the field device, which is included in the device information acquired by the device information acquirer, matches with one of the changes in the state of the field device, included in the estimation rule; and in a case in which the maintenance operation estimator has determined that the change in the state of the field device matches with the one of the changes in the state of the field device, acquire information of the maintenance operation associated with the change in the state of the field device from the estimation rule stored in the estimation rule storage to estimate the maintenance operation performed to the field device; and a parameter designation storage configured to store identification information of the field device and a designation of a parameter of the field device in association with each other, wherein the device information acquirer is configured to acquire a change in the parameter based on the designation of the parameter stored in the parameter designation storage, and wherein the maintenance operation estimator is configured to estimate the maintenance operation based on the change in the parameter of the field device acquired by the device information acquirer. a device information acquirer configured to acquire device information including a change in a state of a field device; an estimation rule storage which stores an estimation rule associating each of multiple changes in the state of the field device with a respective maintenance operation performed to the field device; a maintenance operation estimator configured to: determine whether or not the change in the state of the field device, which is included in the device information acquired by the device information acquirer, matches with one of the changes in the state of the field device, included in the estimation rule; and in a case in which the maintenance operation estimator has determined that the change in the state of the field device matches with the one of the changes in the state of the field device, acquire information of the maintenance operation associated with the change in the state of the field device from the estimation rule stored in the estimation rule storage to estimate the maintenance operation performed to the field device; and determine whether or not the change in the state of the field device, which is included in the device information acquired by the device information acquirer, matches with one of the changes in the state of the field device, included in the estimation rule; and in a case in which the maintenance operation estimator has determined that the change in the state of the field device matches with the one of the changes in the state of the field device, acquire information of the maintenance operation associated with the change in the state of the field device from the estimation rule stored in the estimation rule storage to estimate the maintenance operation performed to the field device; and a parameter designation storage configured to store identification information of the field device and a designation of a parameter of the field device in association with each other, wherein the device information acquirer is configured to acquire a change in the parameter based on the designation of the parameter stored in the parameter designation storage, and wherein the maintenance operation estimator is configured to estimate the maintenance operation based on the change in the parameter of the field device acquired by the device information acquirer.