Polygraphus proximus Blandford (Coleoptera: Curculionidae: Scolytinae) is a non-aggressive monogynous bark beetle. It has caused mass mortality in fir (Abies spp.) forests in the past decade. Although gallery structures made by polygynous beetles may influence their reproductive success, the gallery structure and the number of eggs laid by the monogynous tree-killing bark beetle P. proximus have not been investigated in detail in the natural setting of their native range. We, therefore, investigated the length of mother galleries of P. proximus and the number of eggs oviposited by the beetles in relation to gallery systems with different numbers of arms in Japan. The number of eggs oviposited in two-armed galleries was significantly greater than that observed in one-armed galleries. Additionally, the length of one- or two-armed galleries with no oviposition was significantly shorter than that in galleries consisting of more than two arms. Our results suggest that the optimal number of arms in a gallery system (in terms of the number of eggs oviposited) was two.