A printing apparatus and a conveyance control method are provided, both capable of detecting a print medium conveying state highly precisely in an entire process of conveying the print medium being printed. For this purpose, the surface of the print medium placed on the belt and the surface of the belt are detected to acquire the moving distance or moving speed of the print medium. Based on the moving distance or moving speed thus obtained, the driving of the belt is controlled. Even in a situation where the object being detected switches from the print medium to the belt in the middle of the conveying operation, the measurement of the moving distance can be performed without interruption by the same detection method using the same optical sensor unit.