To improve uniformity of a line width on a substrate by measuring the quantity of light which is split with a semitransparent mirror with a exposure value measuring device so that an actual drawing exposure value is corrected and controlled with an active filter.

The quantity of light which is reflected by a semitransparent mirror 19 just before an objective lens 21 is measured with an exposure value measuring device 20, and, considering control delay time, a light flux shield factor is controlled with an active filter 42 which is provided at a lens 41. Thereby an actual drawing exposure value is controlled. In addition, a change in the exposure value which is caused by scanning a light flux with a catoptric system is measured with the exposure value measuring device 20, and the active filter 42 is controlled while drawing, to control the exposure value. Thereby uniformity of the drawing exposure value is improved and uniformity of a line width on a substrate is improved.