Wildlife managers perform several preventative measures against crop damage caused by sika deer (e.g., fencing and population control); however, these measures have not completely eliminated crop damage, indicating that additional methods should be explored. Therefore, to propose useful agronomical techniques for further decreasing crop damage, we investigated sika deer behavior in apple orchards. The feeding behavior of sika deer was observed using sensor cameras set within apple orchards adjacent to forest. These photographs showed that sika deer mainly used apple orchards in the summer, feeding on both crops (i.e., apple leaves and fruits) and weeds. The estimated 95% confidence interval of the crop-feeding ratio [crop feeding/(crop feeding - weed feeding)] ranged from 0.20 to 0.41, indicating that sika deer more frequently fed on weeds rather than crops. Chemical analyses of the vegetation indicated that only protein content differed among weeds, crops, and forest understory vegetation, with weeds exhibiting the highest protein content. Protein content was probably higher in the apple orchard than in the forest because farmers use nitrogen fertilizer in the apple orchards. We concluded that sika deer are attracted to apple orchards because of the high protein content of the weeds; therefore, weeding might be an effective agronomical technique for reducing crop damage.