Irruptions of ungulate populations have been observed, but little is known of their cause of initiation and termination. We documented an irruption of a naturally colonizing sika deer (Cervus nippon) population on Cape Shiretoko, Shiretoko Peninsula, northeastern Hokkaido, Japan, and we examined limiting factors on population growth. The population increased from 54 deer in 1986 to 592 deer in 1998 (11 to 118 deer/km², respectively) and declined to 177 (35 deer/km²) the following winter of 1999. The intrinsic rate of increase from 1986 to 1998 was 0.19 (95% CI: 0.16 to 0.22). We estimated an annual survival for adult females of 0.92. The ratio of calves to adult females was 76%. We observed a density-correlated reduction in winter food resources. Density-dependent food resources and their interaction with climatic factors were the most important limiting factors for sika deer. The population recovered rapidly following the population crash in 1999 and increased to 512 deer (102 deer/km²) in 2002. We anticipate further increase and a second crash. To confirm whether the population will be regulated naturally and to establish sika deer management policy in Shiretoko National Park, long-term monitoring of the relationship between sika deer and their habitat must be implemented.