A character input device, comprising: an operation unit being a software keyboard on a touchscreen display, having multiple input determination areas for character input, input characters corresponding to the input determination areas being displayed on the touchscreen display, and configured to receive the character input by performing an operation on the touchscreen display, the character input being performed with a flick input by sliding on the touchscreen, wherein the operation unit further comprises a delete button; an operation detector configured to detect a first starting position of a first flick input in a first determination area and an operation direction of the first flick input corresponding to a first input character, configured to detect a deletion operation of the first input character upon pressure of the delete button, and configured detect a second starting position of a second flick input in a second determination area and an operation direction of the second flick input corresponding to a second input character, the second input determination area being different from the first input determination area and corresponding to the second input character; and a determination unit configured to compare the first input character with the second input character, to determine that an input error was made when the deletion operation of the first input character was detected and when the first input character differs from the second input character, to store information in which the first starting position, the first operation direction, and the second input character are associated with one another to count the number of input errors, and, when an input error count exceeds a threshold set for the input error count, to adjust the second input determination area for the second character input by extending the second input determination area in a direction towards the first starting position of the first flick input in the first input determination area such that the extended second input determination area comprises the first starting position of the first flick input, while maintaining the visual appearance of the input characters displayed on the touchscreen display unchanged. an operation unit being a software keyboard on a touchscreen display, having multiple input determination areas for character input, input characters corresponding to the input determination areas being displayed on the touchscreen display, and configured to receive the character input by performing an operation on the touchscreen display, the character input being performed with a flick input by sliding on the touchscreen, wherein the operation unit further comprises a delete button; an operation detector configured to detect a first starting position of a first flick input in a first determination area and an operation direction of the first flick input corresponding to a first input character, configured to detect a deletion operation of the first input character upon pressure of the delete button, and configured detect a second starting position of a second flick input in a second determination area and an operation direction of the second flick input corresponding to a second input character, the second input determination area being different from the first input determination area and corresponding to the second input character; and a determination unit configured to compare the first input character with the second input character, to determine that an input error was made when the deletion operation of the first input character was detected and when the first input character differs from the second input character, to store information in which the first starting position, the first operation direction, and the second input character are associated with one another to count the number of input errors, and, when an input error count exceeds a threshold set for the input error count, to adjust the second input determination area for the second character input by extending the second input determination area in a direction towards the first starting position of the first flick input in the first input determination area such that the extended second input determination area comprises the first starting position of the first flick input, while maintaining the visual appearance of the input characters displayed on the touchscreen display unchanged.