An information processing method of processing data frames flowing over an onboard network including one or more electronic control units that is carried out by an information processing system having a storage unit, the information processing method comprising: a frame collecting step of receiving a plurality of the data frames flowing over the onboard network, obtaining, from each of the multiple data frames, a payload included in the data frame and configured of at least one field, and recording in a reception log stored in the storage unit as one record; and a field extracting step of:: calculating, regarding each of a plurality of payload splitting pattern candidates indicating different regions within payloads of the plurality of data frames, features relating to time-sequence change of values of the payload in the region,from the plurality of records, selecting, from the plurality of payload splitting pattern candidates, a payload splitting pattern indicating a region of a field within the payload, based on the features, and outputting field extracting (S14-S18) results indicating the region indicated by the selected payload splitting pattern, and a category of the field based on the features, wherein the reception log further includes information indicating an order in which the plurality of records have been received, and the features include a first feature representing a count of patterns of the time-sequence change, a second feature representing an occurrence frequency of the time-sequence change, and a third feature representing statistical information relating to an amount of change of the time-sequence change. a frame collecting step of receiving a plurality of the data frames flowing over the onboard network, obtaining, from each of the multiple data frames, a payload included in the data frame and configured of at least one field, and recording in a reception log stored in the storage unit as one record; and a field extracting step of:: calculating, regarding each of a plurality of payload splitting pattern candidates indicating different regions within payloads of the plurality of data frames, features relating to time-sequence change of values of the payload in the region,from the plurality of records, selecting, from the plurality of payload splitting pattern candidates, a payload splitting pattern indicating a region of a field within the payload, based on the features, and outputting field extracting (S14-S18) results indicating the region indicated by the selected payload splitting pattern, and a category of the field based on the features, wherein the reception log further includes information indicating an order in which the plurality of records have been received, and the features include a first feature representing a count of patterns of the time-sequence change, a second feature representing an occurrence frequency of the time-sequence change, and a third feature representing statistical information relating to an amount of change of the time-sequence change. calculating, regarding each of a plurality of payload splitting pattern candidates indicating different regions within payloads of the plurality of data frames, features relating to time-sequence change of values of the payload in the region,from the plurality of records, selecting, from the plurality of payload splitting pattern candidates, a payload splitting pattern indicating a region of a field within the payload, based on the features, and outputting field extracting (S14-S18) results indicating the region indicated by the selected payload splitting pattern, and a category of the field based on the features, wherein the reception log further includes information indicating an order in which the plurality of records have been received, and the features include a first feature representing a count of patterns of the time-sequence change, a second feature representing an occurrence frequency of the time-sequence change, and a third feature representing statistical information relating to an amount of change of the time-sequence change. a first feature representing a count of patterns of the time-sequence change, a second feature representing an occurrence frequency of the time-sequence change, and a third feature representing statistical information relating to an amount of change of the time-sequence change.