An image forming system comprising: an image forming apparatus; and a terminal apparatus connected to the image forming apparatus via a network, characterized in that the image forming apparatus includes: an apparatus main body having an input part to which a printing instruction is input; a sheet feeding cassette stored in the apparatus main body; a manual bypass tray attached to an outer face of the apparatus main body and displaced into an open posture where a sheet feeding becomes possible and into a close posture where the sheet feeding becomes impossible; an opening/closing detection part configured to detect displacements of the manual bypass tray from the open posture to the close posture and from the close posture to the open posture; and a sheet feeding source setting part configured to set the sheet feeding cassette or the manual bypass tray as a sheet feeding source, and the terminal apparatus includes an input device to which a printing instruction is input, wherein when the opening/closing detection part detects that the manual bypass tray is displaced from the close posture to the open posture and the printing instruction is input to the input part, regardless of the sheet feeding source input to the input part, the sheet feeding source setting part sets the manual bypass tray as the sheet feeding source, and when the opening/closing detection part detects that the manual bypass tray is displaced from the close posture to the open posture and the printing instruction is input to the input device, regardless of the sheet feeding source input to the input device, the sheet feeding source setting part sets the sheet feeding cassette as the sheet feeding source. an image forming apparatus; and a terminal apparatus connected to the image forming apparatus via a network, characterized in that the image forming apparatus includes: an apparatus main body having an input part to which a printing instruction is input; a sheet feeding cassette stored in the apparatus main body; a manual bypass tray attached to an outer face of the apparatus main body and displaced into an open posture where a sheet feeding becomes possible and into a close posture where the sheet feeding becomes impossible; an opening/closing detection part configured to detect displacements of the manual bypass tray from the open posture to the close posture and from the close posture to the open posture; and a sheet feeding source setting part configured to set the sheet feeding cassette or the manual bypass tray as a sheet feeding source, and the terminal apparatus includes an input device to which a printing instruction is input, wherein when the opening/closing detection part detects that the manual bypass tray is displaced from the close posture to the open posture and the printing instruction is input to the input part, regardless of the sheet feeding source input to the input part, the sheet feeding source setting part sets the manual bypass tray as the sheet feeding source, and when the opening/closing detection part detects that the manual bypass tray is displaced from the close posture to the open posture and the printing instruction is input to the input device, regardless of the sheet feeding source input to the input device, the sheet feeding source setting part sets the sheet feeding cassette as the sheet feeding source. an apparatus main body having an input part to which a printing instruction is input; a sheet feeding cassette stored in the apparatus main body; a manual bypass tray attached to an outer face of the apparatus main body and displaced into an open posture where a sheet feeding becomes possible and into a close posture where the sheet feeding becomes impossible; an opening/closing detection part configured to detect displacements of the manual bypass tray from the open posture to the close posture and from the close posture to the open posture; and a sheet feeding source setting part configured to set the sheet feeding cassette or the manual bypass tray as a sheet feeding source, and the terminal apparatus includes an input device to which a printing instruction is input, wherein when the opening/closing detection part detects that the manual bypass tray is displaced from the close posture to the open posture and the printing instruction is input to the input part, regardless of the sheet feeding source input to the input part, the sheet feeding source setting part sets the manual bypass tray as the sheet feeding source, and when the opening/closing detection part detects that the manual bypass tray is displaced from the close posture to the open posture and the printing instruction is input to the input device, regardless of the sheet feeding source input to the input device, the sheet feeding source setting part sets the sheet feeding cassette as the sheet feeding source.