A projection system includes a plurality of projection units configured to project an image as a whole; a receiver configured to receive an instruction for shifting reference points that define a projection target area onto which the plurality of projection units project the whole image on an occasion of an initial calibration processing; a storage unit configured to store the reference points of the projection target area used in the initial calibration processing; a controller configured to start a recalibration processing in response to an event; a setting unit configured to reset the projection target area based on the reference points stored in the storage unit on an occasion of the recalibration processing; and a correction coefficient calculator configured to calculate correction coefficients for the plurality of projection units based on the projection target area that is reset by the setting unit.