Males of Lucanus maculifemoratus Motschulsky are markedly variable in size of body and heads. In this study, we investigated the possible causes that affect the development of the male heads and genitalia by comparing variability of weight among body parts and allometry in each part to body size. Genitalia varied least in weight among males, and the frequency distribution of their weight was approximately normal. In contrast, the frequency distribution of head weight exhibited a conspicuous skew. Significant positive allometry was found for heads, whereas genitalia showed significant negative allometry. Heads that are used for fighting are allometrically highly variable, whereas genitalia are highly stable, suggesting variable strategies for obtaining mates while maintaining equally sexually functional genitalia. The low variability and low allometric coefficient with body size for genitalia may indicate that the development mechanism for genitalia is separated from that for other body parts, as a result of sexual selection.