A maskless exposure device including a light source configured to emit an exposure beam, a light modulation element configured to modulate the exposure beam according to an exposure pattern, a projection optical system configured to transfer a modulated exposure beam to a substrate as a beam spot array, a beam measurement part configured to measure a beam data of the beam spot array, and a compensating mask generator configured to generate a compensating mask by utilizing a measured data of the exposure beam for compensating cumulative illumination, wherein the compensating mask generator is configured to turn off left and right beams of a first selected spot beam selected by the beam data, and then to turn off a second selected spot beam.