The invention relates to a device for measuring a mask for microlithography, the device including an imaging device and an autofocusing device. The imaging device comprises an imaging optical unit with a focal plane for imaging the mask, an object stage for mounting the mask, and a movement module for producing a relative movement between object stage and imaging optical unit. The autofocusing device is configured to generate a focusing image by way of the imaging of a focusing structure in a focusing image plane intersecting the focal plane, in which the focusing structure is embodied as a gap. Furthermore, the invention relates to an autofocusing method for a device for measuring a mask for microlithography.