There is provided variable slit width device for a spectroscope which can be used in a narrow space, be widely variable, and be set in slit width with high accuracy. The variable slit width device comprises a pair of freely rotatable rotary parts for pressing inclined guide parts of a pair of slit holders, pressing parts for supporting the pair of rotary parts, linear guides having guide parts formed at the lower surfaces thereof and extending horizontally in a direction crossing with the guide rails at right angles, and pressing parts being fixed to the linear guides, guide rails fitted to the guide parts of the linear guides, feed nuts fixed to the pressing parts, feed screws screwed with the feed nuts, bearings for rotatably supporting the feed screws, preload springs for preloading the feed nuts and the feed screws, a slit driving part for rotatably driving the feed screws, and a housing part for fixing thereto the guide rails, the bearings and the motor.