1. A character input device comprising: a skill estimation unit that estimates a skill of a user in character input operation, based on a history of key operation by the user; a character input interface unit that selects an input character based on a number of times at which a same key has been depressed; a determination period storage unit that stores an original determination period, selected by the user, for determining the character being selected; and a control unit that increases or decreases the determination period according to the skill estimated by the skill estimation unit, wherein during the determination period the character input interface unit judges whether or not a determination operation of the character has been performed.