An ultrasonic flowmeter (1, 100, 200, 300, 400) comprising: a fluid flow path (4, 80, 81) through which a measurement target fluid flows; a pair of ultrasonic transducers (6, 7) disposed upstream and downstream of an upper portion of the fluid flow path and configured to transmit and receive an ultrasonic signal; and a flow rate calculator configured to calculate a flow velocity or flow rate of the measurement target fluid based on a propagation time from when the ultrasonic signal transmitted from one of the pair of ultrasonic transducers propagates through the measurement target fluid to when the ultrasonic signal is received by the other of the pair of ultrasonic transducers, wherein the fluid flow path includes: a main flow path including a plurality of divided flow paths (5a, 5b, 5c, 5d, 85a, 85b, 85c, 86a, 86b, 86c) obtained by dividing a flow path having a rectangular cross-section by a same width; and sub flow paths (22a, 22b, 92a, 92b, 94a, 94b) including an added flow path (24a, 24b, 24c, 24d) having a rectangular cross-section having the same width as the width of the divided flow path and a height lower than the height of the divided flow path, and the flow rate calculator calculates a flow rate of the measurement target fluid flowing through the fluid flow path from the flow velocity or flow rate of the measurement target fluid obtained based on the propagation time, wherein the sub flow paths are disposed within a smallest circle including a flow path cross-section of the main flow path a fluid flow path (4, 80, 81) through which a measurement target fluid flows; a pair of ultrasonic transducers (6, 7) disposed upstream and downstream of an upper portion of the fluid flow path and configured to transmit and receive an ultrasonic signal; and a flow rate calculator configured to calculate a flow velocity or flow rate of the measurement target fluid based on a propagation time from when the ultrasonic signal transmitted from one of the pair of ultrasonic transducers propagates through the measurement target fluid to when the ultrasonic signal is received by the other of the pair of ultrasonic transducers, wherein the fluid flow path includes: a main flow path including a plurality of divided flow paths (5a, 5b, 5c, 5d, 85a, 85b, 85c, 86a, 86b, 86c) obtained by dividing a flow path having a rectangular cross-section by a same width; and sub flow paths (22a, 22b, 92a, 92b, 94a, 94b) including an added flow path (24a, 24b, 24c, 24d) having a rectangular cross-section having the same width as the width of the divided flow path and a height lower than the height of the divided flow path, and a main flow path including a plurality of divided flow paths (5a, 5b, 5c, 5d, 85a, 85b, 85c, 86a, 86b, 86c) obtained by dividing a flow path having a rectangular cross-section by a same width; and sub flow paths (22a, 22b, 92a, 92b, 94a, 94b) including an added flow path (24a, 24b, 24c, 24d) having a rectangular cross-section having the same width as the width of the divided flow path and a height lower than the height of the divided flow path, and the flow rate calculator calculates a flow rate of the measurement target fluid flowing through the fluid flow path from the flow velocity or flow rate of the measurement target fluid obtained based on the propagation time, wherein the sub flow paths are disposed within a smallest circle including a flow path cross-section of the main flow path