A controller includes: a detection unit that detects that a housing unit housing a recording medium has been inserted into an insertion slot for inserting the housing unit; a prediction unit that predicts a vibratory excitation time at which vibrations will occur based on a time detected by the detection unit; a determination unit that determines whether the vibrations will occur based on the vibratory excitation time during a period when an image-forming unit is forming an image on a recording medium supplied from a different supply source for recording media from the housing unit inserted into the insertion slot; and a control unit that controls the image-forming unit to prevent image formation if the determination unit determines that the vibrations will occur during a period when the image-forming unit is forming an image on the recording medium.