A character string input apparatus comprising: a display unit that is adapted to display a keypad image in which a plurality of character keys are arranged, on a screen of a display device; an operation position detection unit that is adapted to detect pressed positions on the screen of the display device; a score calculation unit that is adapted to calculate, for each of the character keys of the keypad image, scores of the pressed positions on the screen of the display device; a score data generation unit that is adapted to generate, for each of the pressed positions on the screen of the display device detected by the operation position detection unit during a character string input period, score data in which the scores for the character keys calculated by the score calculation unit are registered; and an input character string inference unit that is adapted to infer a character string that was input, using the score data generated by the score data generation unit, characterized by a character string database that is adapted to store, for each character string, feature information in which reference data being matrix data, consisting of n rows and M columns or consisting of n columns and M rows where n is the number of character keys arranged in said keypad image and M is a predetermined value, is associated with the character string, wherein the score data generation unit is adapted to generate score data which is matrix data consisting of n rows and m columns or consisting of n columns and m rows, the m rows or m columns corresponding to timings of said character string input period, at which the pressed positions on the screen of the display device are detected by the operation position detection unit, and the n columns or n rows corresponding to said number of the character keys of the keypad image such that the character keys are associated with timings at which the pressed positions on the screen of the display device are detected by the operation position detection unit, element values of the matrix data serving as the scores calculated by the score calculation unit, wherein the score data generation unit is adapted to perform normalization for converting the matrix data consisting of n rows and m columns or consisting of n columns and m rows generated as score data into matrix data being normalized score data and consisting of n rows and M columns or consisting of M rows and n columns, that is independent of the length of the character string input period, wherein the input character string inference unit is adapted to infer that character strings corresponding to reference data whose similarity with normalized score data exceeds a predetermined threshold value are candidates for a character string that has been input. a display unit that is adapted to display a keypad image in which a plurality of character keys are arranged, on a screen of a display device; an operation position detection unit that is adapted to detect pressed positions on the screen of the display device; a score calculation unit that is adapted to calculate, for each of the character keys of the keypad image, scores of the pressed positions on the screen of the display device; a score data generation unit that is adapted to generate, for each of the pressed positions on the screen of the display device detected by the operation position detection unit during a character string input period, score data in which the scores for the character keys calculated by the score calculation unit are registered; and an input character string inference unit that is adapted to infer a character string that was input, using the score data generated by the score data generation unit, characterized by a character string database that is adapted to store, for each character string, feature information in which reference data being matrix data, consisting of n rows and M columns or consisting of n columns and M rows where n is the number of character keys arranged in said keypad image and M is a predetermined value, is associated with the character string, wherein the score data generation unit is adapted to generate score data which is matrix data consisting of n rows and m columns or consisting of n columns and m rows, the m rows or m columns corresponding to timings of said character string input period, at which the pressed positions on the screen of the display device are detected by the operation position detection unit, and the n columns or n rows corresponding to said number of the character keys of the keypad image such that the character keys are associated with timings at which the pressed positions on the screen of the display device are detected by the operation position detection unit, element values of the matrix data serving as the scores calculated by the score calculation unit, wherein the score data generation unit is adapted to perform normalization for converting the matrix data consisting of n rows and m columns or consisting of n columns and m rows generated as score data into matrix data being normalized score data and consisting of n rows and M columns or consisting of M rows and n columns, that is independent of the length of the character string input period, wherein the input character string inference unit is adapted to infer that character strings corresponding to reference data whose similarity with normalized score data exceeds a predetermined threshold value are candidates for a character string that has been input.