A printing apparatus which includes at least a first tray and a second tray as a tray which accommodates a printing sheet, and can be mounted with a sheet supply unit which supplies the printing sheet by being switched to any one of the first tray and the second tray, wherein the sheet supply unit is a feeding roller for feeding a sheet in each of the trays towards a recording head of the printing apparatus, the apparatus further comprising: a storage unit adapted to store information related to the printing sheet in association with the one of the first and second trays that accommodates the printing sheet; a display unit adapted to inform of a message by displaying the message; a reception unit adapted to receive a print job which instructs printing; a determination unit adapted to determine that printing is possible when information of a first printing sheet which is accommodated in the first tray, and information of a printing sheet instructed by the print job are the same, and to determine that printing is possible by switching to the second tray, when information of a second printing sheet which is accommodated in the second tray, and information of the printing sheet instructed by the job are the same; a printing unit adapted to perform printing on the printing sheet based on the print job; and a control unit adapted to control printing using the printing unit based on a determination result of the determination unit and to cause the display unit to display a message which asks for switching to the second tray when the determination unit determines that printing is possible by switching to the second tray; and characterized by a detection unit adapted to detect a mounting state of the sheet supply unit, and the first tray which can supply the printing sheet in the mounted sheet supply unit. a storage unit adapted to store information related to the printing sheet in association with the one of the first and second trays that accommodates the printing sheet; a display unit adapted to inform of a message by displaying the message; a reception unit adapted to receive a print job which instructs printing; a determination unit adapted to determine that printing is possible when information of a first printing sheet which is accommodated in the first tray, and information of a printing sheet instructed by the print job are the same, and to determine that printing is possible by switching to the second tray, when information of a second printing sheet which is accommodated in the second tray, and information of the printing sheet instructed by the job are the same; a printing unit adapted to perform printing on the printing sheet based on the print job; and a control unit adapted to control printing using the printing unit based on a determination result of the determination unit and to cause the display unit to display a message which asks for switching to the second tray when the determination unit determines that printing is possible by switching to the second tray; and characterized by a detection unit adapted to detect a mounting state of the sheet supply unit, and the first tray which can supply the printing sheet in the mounted sheet supply unit.