A method of reducing an odor of composite resin particles comprising the step of: fluidizing the composite resin particles, which contain a polyolefin-based resin and a polystyrene-based resin and which are used for producing an expanded molded article, in a container with a gas that is blown thereinto from its bottom and that has a temperature of (T−40)° C. to (T−10)° C. (T is a softening temperature of the composite resin particles), to reduce the odor generated from a raw material of the composite resin particles and generated according to a method of the composite resin particles, wherein the composite resin particles whose odor is reduced are impregnated with a blowing agent; and the resultant is pre-expanded and then subject to an in-mold forming to make the expanded molded article have an odor strength of 3 or less in average value in odor test in which odor of an isovaleric acid diluted 100000 times is defined as 3, which means a reference odor, within 0 to 5 levels of the odor strength.