To obtain fixed quantity of exposure at each radial position from the center of a slit disc, and to quickly fetch confocal pictures without revolving the slit disc.

Slits to be formed on a slit disc 13 of a confocal device 10 are not radially formed from the center of the slit disc 13 but formed so as to be increased almost in proportion to the increase of radial positions 1-5. Thus, mean exposure R can be obtained at the radial positions 1-5 of the slit disc 13. Therefore, it is not necessary to revolve the slit disc 13 by a driving motor 18, and it is possible to fetch almost mean exposure R at the radial positions 1-5, and to quickly fetch the confocal pictures of a sample 16 by moving the slit disc 13 to the revolving direction a little.