Players with a wide range of levels or attack strength can enjoy a time-limited group battle, and the participation rate can be expected to improve throughout the set time slot. A battle time managing unit refers to a timer and a memory resource to manage the time of a battle between groups. A battle condition extraction unit refers to the memory resource and checks whether to change a battle condition during the time of the battle. A target changing unit refers to the memory resource to set a target of change based on the battle condition. A battle condition changing unit changes the battle condition for the target of change that was selected by the target changing unit. Based on the changed battle condition, a battle rendering unit executes rendition processing for the game related to the battle.