A fluid measuring apparatus comprising: at least one pair of ultrasonic wave transmitting and receiving means which is configured to transmit and receive an ultrasonic wave to and from a fluid flowing in a pipe; a period measuring unit which is configured to measure a propagation period of the ultrasonic wave transmitted and received by the ultrasonic wave transmitting and receiving means; and a flow velocity measuring unit which is configured to derive flow velocity of the fluid from the propagation period of the ultrasonic wave measured by the period measuring unit, wherein: one of the pair of ultrasonic wave transmitting and receiving means is disposed in an upstream side of the pipe, and the other is disposed in a downstream side of the pipe; the period measuring unit is configured to measure a first propagation period during which the ultrasonic wave propagates from the ultrasonic wave transmitting and receiving means in the upstream side to the ultrasonic wave transmitting and receiving means in the downstream side, and a second propagation period during which ultrasonic wave propagates from the ultrasonic wave transmitting and receiving means in the downstream side to the ultrasonic wave transmitting and receiving means in the upstream side; and the flow velocity measuring unit is configured to derive flow velocity of the fluid so as to cancel errors using flow velocity Va which is derived by measuring the second propagation period after measuring the first propagation period, and flow velocity Vb which is derived by measuring the first propagation period after measuring the second propagation period, characterized in that: the fluid measuring apparatus comprises a plurality of the pairs of ultrasonic wave transmitting and receiving means, wherein the first propagation period and the second propagation period are measured in each of the pairs of ultrasonic wave transmitting and receiving means; and each of the plurality of the pairs of ultrasonic wave transmitting and receiving means is weighted; flow velocities which are to be measured by each of the pairs are set, based on their respective amounts of weighting, to one of the flow velocity Va and the flow velocity Vb; and flow velocity of the fluid is derived with weight addition. at least one pair of ultrasonic wave transmitting and receiving means which is configured to transmit and receive an ultrasonic wave to and from a fluid flowing in a pipe; a period measuring unit which is configured to measure a propagation period of the ultrasonic wave transmitted and received by the ultrasonic wave transmitting and receiving means; and a flow velocity measuring unit which is configured to derive flow velocity of the fluid from the propagation period of the ultrasonic wave measured by the period measuring unit, wherein: one of the pair of ultrasonic wave transmitting and receiving means is disposed in an upstream side of the pipe, and the other is disposed in a downstream side of the pipe; the period measuring unit is configured to measure a first propagation period during which the ultrasonic wave propagates from the ultrasonic wave transmitting and receiving means in the upstream side to the ultrasonic wave transmitting and receiving means in the downstream side, and a second propagation period during which ultrasonic wave propagates from the ultrasonic wave transmitting and receiving means in the downstream side to the ultrasonic wave transmitting and receiving means in the upstream side; and the flow velocity measuring unit is configured to derive flow velocity of the fluid so as to cancel errors using flow velocity Va which is derived by measuring the second propagation period after measuring the first propagation period, and flow velocity Vb which is derived by measuring the first propagation period after measuring the second propagation period, one of the pair of ultrasonic wave transmitting and receiving means is disposed in an upstream side of the pipe, and the other is disposed in a downstream side of the pipe; the period measuring unit is configured to measure a first propagation period during which the ultrasonic wave propagates from the ultrasonic wave transmitting and receiving means in the upstream side to the ultrasonic wave transmitting and receiving means in the downstream side, and a second propagation period during which ultrasonic wave propagates from the ultrasonic wave transmitting and receiving means in the downstream side to the ultrasonic wave transmitting and receiving means in the upstream side; and the flow velocity measuring unit is configured to derive flow velocity of the fluid so as to cancel errors using flow velocity Va which is derived by measuring the second propagation period after measuring the first propagation period, and flow velocity Vb which is derived by measuring the first propagation period after measuring the second propagation period, characterized in that: the fluid measuring apparatus comprises a plurality of the pairs of ultrasonic wave transmitting and receiving means, wherein the first propagation period and the second propagation period are measured in each of the pairs of ultrasonic wave transmitting and receiving means; and each of the plurality of the pairs of ultrasonic wave transmitting and receiving means is weighted; flow velocities which are to be measured by each of the pairs are set, based on their respective amounts of weighting, to one of the flow velocity Va and the flow velocity Vb; and flow velocity of the fluid is derived with weight addition.