A printer is provided with: a detector, operable to output an output signal in accordance with a state of a printing medium transported within the printer; and a control signal generator, operable to generate a control signal in accordance with a level of the output signal relative to a threshold. In order to control the printer, the output signal is detected. A timing signal indicative of a timing to judge the state of the printing medium is detected. The control signal is generated with at least two of: a first method, in which the control signal is made valid when the level of the output signal reaches the threshold after the timing signal is detected, and the control signal is unchanged until when a first time period is elapsed from when the control signal is made valid; a second method, in which the control signal is made valid when the level of the output signal reaches the threshold at least once after the timing signal is detected, and the control signal is unchanged until when a next timing signal is detected; and a third method, in which the control signal is made valid in accordance with a condition that the level of the output signal lastly reaches the threshold before a second time period is elapsed from when the timing signal is detected, and the control signal is unchanged until when a next timing signal is detected.