A method for producing a Group III-V semiconductor nanoparticle by flow reaction, including: introducing a solution of compound containing Group III element into a first flow channel, introducing a solution of compound containing Group V element into a second flow channel, and combining the solutions to produce nanoparticles, in which the combining portion is constituted by a multi-layered tubular mixer, one of the solutions is allowed to flow through a flow channel in the smallest tube of the mixer, and the other of the solutions is allowed to flow through a flow channel adjacent to the flow channel in the smallest tube, and a value of a ratio of linear velocity of the solution flowing in the flow channel adjacent to the flow channel in the smallest tube to linear velocity of the solution flowing in the flow channel in the smallest tube is a specific value.