Bark beetles (Curculionidae: Scolytinae) cause widespread tree mortality and change forest structure, composition, and function; however, few data are available on bark beetle infestation of true fir forests. Although Polygraphus proximus Brandford (Coleoptera: Scolytidae) causes rapid degradation of large areas of natural fir forests in Russia, they are not present in large numbers or cause mass mortality in central Japanese subalpine forests dominated by Abies veitchii. Additionally, little is known about its preferred tree trunk size, which is a crucial parameter affecting bark beetle colonization and tree mortality. We investigated the rate of P. proximus attack on A. veitchii and the association between resulting tree mortality and tree diameter at two sites on Honshu and Hokkaido, Japan. Polygraphus proximus colonized and utilized standing A. veitchii trees and logs. The A. veitchii mortality rate was 33.8% in Honshu and 59.1% in Hokkaido. The proportion of trees with holes was significantly higher in smaller-diameter trees than in larger-diameter trees, and smaller-diameter trees had significantly more holes than larger-diameter trees, suggesting that P. proximus prefers smaller-diameter trees. Additionally, larger-diameter trees showed higher survival of P. proximus attacks. Preferential colonization of smaller-diameter trees by P. proximus and their higher mortality rate might pose a threat to regeneration of A. veitchii forests in Japanese subalpine areas.