A field device configured to measure a plurality of density values of a fluid as a measured value, the field device comprising: a slug flow detector configured to detect an occurrence of a slug flow; and a controller configured to hold a hold value of the measured value if the occurrence of the slug flow is detected by the slug flow detector, the hold value being a value based on a plurality of density values in a predetermined period from a predetermined time before a time at which the slug flow has been detected; wherein the controller comprises: an integrated value storage configured to store an integrated value of the plurality of density values in the predetermined period; and a corrector configured to calculate an average value based on the plurality of density values in the predetermined period on the basis of the integrated value stored in the integrated value storage; wherein a start time of the predetermined period for starting a calculation of the average value is a time after a detection of the occurrence of the slug flow is cancelled, an end time of the predetermined period for ending the calculation of the average value is a time which is a predetermined time before a subsequent slug flow is detected, the controller is configured to hold the average value as the hold value from when the occurrence of the slug flow is detected to the start time, the corrector is configured to calculate the average value based on the plurality of density values in the predetermined period from the start time to the end time; wherein the integrated value storage includes a plurality of integration storage regions (A to E), each of the integration storage regions (A to E) includes a plurality of integration regions (100 to 140, 200 to 240, 300 to 340, 400 to 440, 500 to 540) for storing the integrated value, and the corrector is configured to move the integrated value stored in each of the integration regions (100 to 140, 200 to 240, 300 to 340, 400 to 440, 500 to 540) to an integration region corresponding to a next time interval for every predetermined time interval. a slug flow detector configured to detect an occurrence of a slug flow; and a controller configured to hold a hold value of the measured value if the occurrence of the slug flow is detected by the slug flow detector, the hold value being a value based on a plurality of density values in a predetermined period from a predetermined time before a time at which the slug flow has been detected; wherein the controller comprises: an integrated value storage configured to store an integrated value of the plurality of density values in the predetermined period; and a corrector configured to calculate an average value based on the plurality of density values in the predetermined period on the basis of the integrated value stored in the integrated value storage; wherein a start time of the predetermined period for starting a calculation of the average value is a time after a detection of the occurrence of the slug flow is cancelled, an end time of the predetermined period for ending the calculation of the average value is a time which is a predetermined time before a subsequent slug flow is detected, the controller is configured to hold the average value as the hold value from when the occurrence of the slug flow is detected to the start time, the corrector is configured to calculate the average value based on the plurality of density values in the predetermined period from the start time to the end time; an integrated value storage configured to store an integrated value of the plurality of density values in the predetermined period; and a corrector configured to calculate an average value based on the plurality of density values in the predetermined period on the basis of the integrated value stored in the integrated value storage; wherein a start time of the predetermined period for starting a calculation of the average value is a time after a detection of the occurrence of the slug flow is cancelled, an end time of the predetermined period for ending the calculation of the average value is a time which is a predetermined time before a subsequent slug flow is detected, the controller is configured to hold the average value as the hold value from when the occurrence of the slug flow is detected to the start time, the corrector is configured to calculate the average value based on the plurality of density values in the predetermined period from the start time to the end time; the integrated value storage includes a plurality of integration storage regions (A to E), each of the integration storage regions (A to E) includes a plurality of integration regions (100 to 140, 200 to 240, 300 to 340, 400 to 440, 500 to 540) for storing the integrated value, and the corrector is configured to move the integrated value stored in each of the integration regions (100 to 140, 200 to 240, 300 to 340, 400 to 440, 500 to 540) to an integration region corresponding to a next time interval for every predetermined time interval.