A fluid measuring apparatus comprising: an irradiating device configured to irradiate a fluid with light; a light receiving device configured to receive light scattered by the fluid; a detecting device configured to detect a received light intensity indicated by a received light signal of said light receiving device; a calculating device configured to calculate estimated fluid information indicating a flow volume or a flow velocity of the fluid, on the basis of a detection result of said detecting device and the received light signal of said light receiving device; and an analyzing device configured to output an average frequency signal on the basis of the received light signal, wherein the detecting device is configured to detect that a change amount of received light intensity indicated by a received light signal of said light receiving device is greater than or equal to a predetermined value, characterized in that the detecting device is configured to detect a backflow of the fluid if the change amount of received light intensity indicated by the received light signal is greater than or equal to the predetermined value, wherein either said calculating device is configured (i) to calculate the estimated fluid information on the basis of the average frequency signal in a period other than a backflow period in which the backflow of the fluid is detected, and (ii) to calculate the estimated fluid information on the basis of a corrected average frequency signal obtained by correcting the average frequency signal in the backflow period; or said calculating device is configured to calculate first fluid information on the basis of the average frequency signal and second fluid information by correcting the first fluid information, (i) to output the first fluid information as the estimated fluid information in a period other than a backflow period in which the backflow of the fluid is detected, and (ii) to output the second fluid information as the estimated fluid information in the backflow period. an irradiating device configured to irradiate a fluid with light; a light receiving device configured to receive light scattered by the fluid; a detecting device configured to detect a received light intensity indicated by a received light signal of said light receiving device; a calculating device configured to calculate estimated fluid information indicating a flow volume or a flow velocity of the fluid, on the basis of a detection result of said detecting device and the received light signal of said light receiving device; and an analyzing device configured to output an average frequency signal on the basis of the received light signal, wherein the detecting device is configured to detect that a change amount of received light intensity indicated by a received light signal of said light receiving device is greater than or equal to a predetermined value, characterized in that the detecting device is configured to detect a backflow of the fluid if the change amount of received light intensity indicated by the received light signal is greater than or equal to the predetermined value, wherein either said calculating device is configured (i) to calculate the estimated fluid information on the basis of the average frequency signal in a period other than a backflow period in which the backflow of the fluid is detected, and (ii) to calculate the estimated fluid information on the basis of a corrected average frequency signal obtained by correcting the average frequency signal in the backflow period; or said calculating device is configured to calculate first fluid information on the basis of the average frequency signal and second fluid information by correcting the first fluid information, (i) to output the first fluid information as the estimated fluid information in a period other than a backflow period in which the backflow of the fluid is detected, and (ii) to output the second fluid information as the estimated fluid information in the backflow period.