A multiphase flowmeter for measurement of a flow quantity of liquid, gas and solid particles which are mixed in a multiphase fluid, the multiphase flowmeter comprising: a plurality of detectors (21, 22, 31, 32) configured to detect gas bubbles which are included in a fluid, the plurality of the detectors (21, 22, 31, 32) being disposed on an outside wall of a pipe line (11, 12) in which the fluid flows, the plurality of detectors (21, 22, 31, 32) being disposed along a flow direction of the fluid; and a calculator configured to determine whether the gas bubble exists in the fluid or not in accordance with the detection result of the plurality of the detectors (21, 22, 31, 32), wherein said plurality of detectors (21, 22, 31, 32) comprises: a first detector (21 and 22) configured to transmit a first ultrasonic wave to a gas bubble in the fluid, the first detector (21 and 22) being configured to output a first signal based on the first ultrasonic wave reflected by the gas bubble in the fluid; a second detector (31 and 32) configured to transmit a second ultrasonic wave to a gas bubble in the fluid, the second detector (31 and 32) being configured to output a second signal based on the second ultrasonic wave reflected by the gas bubble in the fluid; and wherein the calculator is configured to receive the first signal output from the first detector (21 and 22) and to receive the second signal output from the second detector (31 and 32); the calculator is configured to calculate a diameter of the gas bubble and a number density of the gas bubble per unit volume based on the first signal and the second signal, and the calculator is configured to calculate a quantity of the gas bubble based on the diameter of the gas bubble and the number density of the gas bubble per unit volume; wherein the pipeline comprises a horizontal pipe line which is connected to a vertical pipeline, the first detector (21, 22) is disposed on the vertical pipeline, the second detector (31, 32) is disposed on the horizontal pipeline, and wherein the calculator is configured to calculate a first velocity which is a velocity of the gas bubble flowing in the vertical pipe line based on the first signal output from the first detector (21 and 22), calculate a second velocity which is a velocity of the gas bubble flowing in the horizontal pipe line based on the second signal output from the second detector (31 and 32), and calculate the diameter of the gas bubble based on a difference between the first velocity and the second velocity. a plurality of detectors (21, 22, 31, 32) configured to detect gas bubbles which are included in a fluid, the plurality of the detectors (21, 22, 31, 32) being disposed on an outside wall of a pipe line (11, 12) in which the fluid flows, the plurality of detectors (21, 22, 31, 32) being disposed along a flow direction of the fluid; and a calculator configured to determine whether the gas bubble exists in the fluid or not in accordance with the detection result of the plurality of the detectors (21, 22, 31, 32), wherein said plurality of detectors (21, 22, 31, 32) comprises: a first detector (21 and 22) configured to transmit a first ultrasonic wave to a gas bubble in the fluid, the first detector (21 and 22) being configured to output a first signal based on the first ultrasonic wave reflected by the gas bubble in the fluid; a second detector (31 and 32) configured to transmit a second ultrasonic wave to a gas bubble in the fluid, the second detector (31 and 32) being configured to output a second signal based on the second ultrasonic wave reflected by the gas bubble in the fluid; and wherein the calculator is configured to receive the first signal output from the first detector (21 and 22) and to receive the second signal output from the second detector (31 and 32); a first detector (21 and 22) configured to transmit a first ultrasonic wave to a gas bubble in the fluid, the first detector (21 and 22) being configured to output a first signal based on the first ultrasonic wave reflected by the gas bubble in the fluid; a second detector (31 and 32) configured to transmit a second ultrasonic wave to a gas bubble in the fluid, the second detector (31 and 32) being configured to output a second signal based on the second ultrasonic wave reflected by the gas bubble in the fluid; and wherein the calculator is configured to receive the first signal output from the first detector (21 and 22) and to receive the second signal output from the second detector (31 and 32); the calculator is configured to calculate a diameter of the gas bubble and a number density of the gas bubble per unit volume based on the first signal and the second signal, and the calculator is configured to calculate a quantity of the gas bubble based on the diameter of the gas bubble and the number density of the gas bubble per unit volume; wherein the pipeline comprises a horizontal pipe line which is connected to a vertical pipeline, the first detector (21, 22) is disposed on the vertical pipeline, the second detector (31, 32) is disposed on the horizontal pipeline, and wherein the calculator is configured to calculate a first velocity which is a velocity of the gas bubble flowing in the vertical pipe line based on the first signal output from the first detector (21 and 22), calculate a second velocity which is a velocity of the gas bubble flowing in the horizontal pipe line based on the second signal output from the second detector (31 and 32), and calculate the diameter of the gas bubble based on a difference between the first velocity and the second velocity.