A flow channel structure for forming nano-sized lipid particles or micelles, wherein in the flow channel structure, a mutually independent first inlet channel that is configured to introduce a first fluid and a second inlet channel that is configured to introduce a second fluid join together and have respectively fixed lengths on the upstream side thereof and a single dilution flow channel is formed towards the downstream side thereof, the dilution flow channel has a bent flow channel site that is bent two-dimensionally in at least a portion thereof, and the bent flow channel site is such that, in the case the axial direction of the dilution flow channel upstream therefrom or the direction in which it extends is defined as the X direction, the widthwise direction of the dilution flow channel that perpendicularly intersects with this X direction is defined as the Y direction, and the flow channel width of the dilution flow channel upstream therefrom is defined as y 0, at least two or more structural elements, which define the flow channel width of the dilution flow channel by alternately protruding from both side surfaces of the dilution flow channel in opposition to the Y direction towards the center of the flow channel at a specific height h 1, h 2,... of 1/2y 0 or more and less than 1y 0 in the Y direction (+Y direction or -Y direction) and at a specific width x 1, x 2,... in the X direction, are provided at specific intervals d 1, d 2,...., wherein the flow channel width y 0 is 20 μm to 1000 μm, the width of each structural element x 1, x 2,... is 20 μm to 1000 μm, the height of each structural element h 1, h 2,... is 1/2y 0 or more to 3/4y 0 or less, and the interval d 1, d 2,... between each structural element is 20 μm to 1000 μm, and wherein the structural elements are provided from 10 to 100. wherein in the flow channel structure, a mutually independent first inlet channel that is configured to introduce a first fluid and a second inlet channel that is configured to introduce a second fluid join together and have respectively fixed lengths on the upstream side thereof and a single dilution flow channel is formed towards the downstream side thereof, the dilution flow channel has a bent flow channel site that is bent two-dimensionally in at least a portion thereof, and the bent flow channel site is such that, in the case the axial direction of the dilution flow channel upstream therefrom or the direction in which it extends is defined as the X direction, the widthwise direction of the dilution flow channel that perpendicularly intersects with this X direction is defined as the Y direction, and the flow channel width of the dilution flow channel upstream therefrom is defined as y 0, at least two or more structural elements, which define the flow channel width of the dilution flow channel by alternately protruding from both side surfaces of the dilution flow channel in opposition to the Y direction towards the center of the flow channel at a specific height h 1, h 2,... of 1/2y 0 or more and less than 1y 0 in the Y direction (+Y direction or -Y direction) and at a specific width x 1, x 2,... in the X direction, are provided at specific intervals d 1, d 2,...., wherein the flow channel width y 0 is 20 μm to 1000 μm, the width of each structural element x 1, x 2,... is 20 μm to 1000 μm, the height of each structural element h 1, h 2,... is 1/2y 0 or more to 3/4y 0 or less, and the interval d 1, d 2,... between each structural element is 20 μm to 1000 μm, and wherein the structural elements are provided from 10 to 100.