A Coriolis flowmeter (1; 51) configured to drive flow tubes (2; 52) and detect a phase difference and/or a vibration frequency proportional to a Coriolis force acting on each of the flow tubes (2; 52), to thereby determine a mass flow rate and/or density of a fluid to be measured, which flows through each of the flow tubes (2; 52), the Coriolis flowmeter (1; 51) comprising manifolds (8; 56) arranged at both end portions of each of the flow tubes (2; 52), wherein a number of the flow tubes (2; 52) is set to two, wherein a channel (15; 57) of each of the manifolds (8; 56) comprises, as shaping portions therefor: a pipe-side opening portion arranged close to a pipe to which the Coriolis flowmeter (1; 51) is to be installed; tube-side opening portions arranged close to the flow tubes (2; 52), a number of the tube-side opening portions being equal to the two flow tubes (2; 52); and a channel branching portion to be branched corresponding to the two flow tubes (2; 52), wherein areas of branching portion sectional shapes of the channel branching portion are set equal to each other, wherein each of the branching portion sectional shapes of the channel branching portion has a D-shape, wherein the shape of each of the tube-side opening portions has a circular shape, characterized in that a channel sectional area in a range from the channel branching portion toward the tube-side opening portions is set so as to be prevented from becoming larger than the areas of the branching portion sectional shapes, the channel sectional area in a range from the channel branching portion toward the tube-side opening portions linearly decreases from the areas of the branching portion sectional shapes; and the sectional shape (26, 27, 28, 29, 30, 31, 32, 33, 34, 35) of the channel (15; 57) in the range from the channel branching portion toward the tube-side opening portions is changed from the D-shape, to a substantially D-shape, to a substantially circular shape, and to the circular shape. the Coriolis flowmeter (1; 51) comprising manifolds (8; 56) arranged at both end portions of each of the flow tubes (2; 52), wherein a number of the flow tubes (2; 52) is set to two, wherein a channel (15; 57) of each of the manifolds (8; 56) comprises, as shaping a pipe-side opening portion arranged close to a pipe to which the Coriolis flowmeter (1; 51) is to be installed; tube-side opening portions arranged close to the flow tubes (2; 52), a number of the tube-side opening portions being equal to the two flow tubes (2; 52); and a channel branching portion to be branched corresponding to the two flow tubes (2; 52), wherein areas of branching portion sectional shapes of the channel branching portion are set equal to each other, wherein each of the branching portion sectional shapes of the channel branching portion has a D-shape, wherein the shape of each of the tube-side opening portions has a circular shape, characterized in that a channel sectional area in a range from the channel branching portion toward the tube-side opening portions is set so as to be prevented from becoming larger than the areas of the branching portion sectional shapes, the channel sectional area in a range from the channel branching portion toward the tube-side opening portions linearly decreases from the areas of the branching portion sectional shapes; and the sectional shape (26, 27, 28, 29, 30, 31, 32, 33, 34, 35) of the channel (15; 57) in the range from the channel branching portion toward the tube-side opening portions is changed from the D-shape, to a substantially D-shape, to a substantially circular shape, and to the circular shape.