Provided is a composite including copper nanoparticles or copper(I) oxide nanoparticles and a thioether-containing organic compound represented by X(OCH2CHR1)nOCH2CH(OH)CH2SZ [X represents an alkyl group; R1 represents a hydrogen atom or a methyl group; n represents an integer of 2 to 100; R1 is independent between repeating units and may be the same or different; and Z represents an alkyl group, an allyl group, an aryl group, an arylalkyl group, —R2—OH, —R2—NHR3, or —R2—(COR4)m (where R2 represents a saturated hydrocarbon group; R3 represents a hydrogen atom, an acyl group, an alkoxycarbonyl group, or a benzyloxycarbonyl group; R4 represents a hydroxy group, an alkyl group, or an alkoxy group; and m represents 1 to 3)]. Provided is a method for producing a composite of an organic compound and copper nanoparticles or a composite of an organic compound and copper(I) oxide nanoparticles, the method including reducing a copper compound in the presence of a thioether-containing organic compound represented by the general formula (1) above.