This report describes a search for a new method for assistance in assignment of complex molecular spectra without obviously recognizable regular patterns, on which traditional methods of spectral assignment have depended. We propose “second difference method reinforced by grouping,” a computer-aided technique for picking out regular patterns buried in a list of observed line wavenumbers which look like randomly distributed. The results of tests suggest that this technique may be succesfully applied even to the cases in which the method of Loomis-Wood diagram would fail. Superiority of the present method to the Loomis-Wood method in immunity against the interference by the overlap of unrelated spectral bands is also demonstrated.