PROBLEM TO BE SOLVED: To provide a multistage split flow path type mixer having a micro-flow path and capable of quickly, uniformly mixing even a high-viscosity fluid under a laminar flow condition.SOLUTION: A multistage split flow path type micromixer for mixing fluids A and B includes: a fluid A-splitting part for evenly splitting the fluid A into a plurality of tubular flow paths by generating uniform pressure loss after the fluid A flows in the mixer; a fluid B-splitting part for evenly splitting the fluid B into the plurality of tubular flow paths by generating uniform pressure loss after the fluid B flows on the downstream side of the fluid A-splitting part in the mixer; double tube type mixing parts composing double tubular mixer structures which are provided in the same numbers as the fluid A split numbers on the downstream side of the fluid B-splitting part and have the fluids A and B as the inside and outside fluids, respectively; and individual contraction flow parts which are provided in the same
