PURPOSE: To correctly and accurately detect a character string even when there is a contact character which is difficult to detect until now in a complicated scene picture by limiting a back ground area by maximum or minimum value projection, e.g. within an area set to a detected reference character and furthermore, similarly detecting the character by projection in the back ground area.

CONSTITUTION: In a signboard 31 existing in the scene picture, e.g. there is the character string 32 consisting of plural characters. An input picture like this is stored in a gradation picture memory as gradation data of 512×512 picture element, e.g. and the contrast of the stored picture is detected by a binarization processing part so as to obtain a next binary picture. Then, if numerals 2 to 4 are detected as the reference characters by a reference character detecting part, a character string detection part analyzes a surrounding gradation pattern and segments another numeral 5 based on the areas of the reference characters, next. An area 33 is extended by a width W in the direction of the character string with respect to the numerals 2 to 4, e.g. and maximum projection is measured to obtain the area of the signboard 31 to detect the characters.