To provide an epi-illumination unit capable of adjusting and arranging an optional optical element at an optional position on the optical axis of epi- illumination and being easily inserted and extracted into/from the optical axis of epi-illumination.

This epi-illumination unit insertable into a slot part of an epi- illuminating device with a field of view diaphragm unit or an aperture diaphragm unit mounted thereon is provided with a focus mechanism for making a holding frame 44 for holding the optical element movable in the direction of the optical axis epi-illumination with the rotation of a focus shaft 48, an insertion and extraction mechanism having an inserting and extracting part 43 for holding the holding frame 44 and for making the holding frame 44 movable perpendicularly with respect to the optical axis of epi-illumination, a centering mechanism having a centering frame 42 for making the inserting and extracting part 43 movable within a surface perpendicular to the optical axis of epi-illumination, and an outer frame 36 for integrally housing the mechanisms and having a slot shape so as to be inserted and positioned into the slot part of the epi-illuminating device.