A numerical controller corrects a tool compensation value for a machine tool provided with a tool for machining a workpiece based on an NC machining program. The numerical controller makes it unnecessary for a person in charge of input to be conscious of the sign of the tool compensation value and prevents a tool compensation value input error by providing an input button for bringing a workpiece and a tool cutting edge close to each other by a specified amount (+input advance) and an input button for causing the workpiece and the tool cutting edge to be away from each other by a specified amount (+input retreat) at the time of inputting the tool compensation value.