An optical sensor is transported to a position above a marker on a tray, where it irradiates the marker and measures the reflected light so as to obtain a reference value. A threshold value is determined based on the reference value. The optical sensor is transported above the tray, while emitting light and measuring reflected light. The measured amount is compared with the threshold value. A marker position is identified based on the comparison and the threshold value. A start print position is based on the marker position.