Provided are an information display device and a character string converting method that improve operability at the time of changing a character string on a display screen.The information display device is configured to have: a display screen 12 for displaying a character string; a touch panel controller 27 that detects an operation position; a character string selecting part 34 that, when two different operation positions are detected, on the basis of a result of the detection of the operation positions, selects a character string on the display screen 12; and a character string converting part 36 that, when a distance between the two operation positions is changed, converts the character string selected by the character string selecting part 34 depending on whether the change is a change that causes the distance between the operation positions to be narrowed or enlarged. For example, the character string converting part 36 converts a full-width character in the character string to a half-width character in the case where the change in distance between the two operation positions is the change that causes the distance between the operation positions to be narrowed, and converts a half-width character in the character string to a full-width character in the case where the change is the change that causes the distance between the operation positions to be enlarged.