Abstract: A numerical controller that achieves detection of a lifetime of a belt in a machine whose axis and motor are interconnected by the belt includes a axis pulse acquisition unit obtaining a pulse of a axis provided in a machining tool to be controlled, a motor pulse acquisition unit obtaining a pulse of the motor driving the axis, a axis revolution speed calculation unit calculating a revolution speed of the axis based on the obtained axis pulse, a motor revolution speed calculation unit calculating a revolution speed of the motor from the obtained motor pulse, a substantial slip distance calculation unit calculating a substantial slip distance of the belt from the axis revolution speed and the motor revolution speed, and a belt lifetime determination unit determining that the belt has reached the end of its lifetime when the substantial slip distance of the belt exceeds a predetermined threshold.