PURPOSE: To enable easy changeover of stereoscopic observation and flat observation by constituting simply the titled microscope wherein a luminous flux splitting element formed with a total reflection face on the half part on one side relative with the optical axis and a total transmission surface on the other half is disposed attachably and detachably in the optical path.

CONSTITUTION: A luminous flux splitting prism 16 is changeable and movable to positions A, B, C in the arrow directions. When the prism is adjusted to an optical axis A, the luminous flux R for a right eye viewing system of the light illuminating a sample 4 is totally reflected by the 1st reflection face 16a and is observed by an eyepiece lens 11' of the right eye viewing system. The luminous flux L for a left eye viewing system transmits the 1st reflection surface 16a and is then totally reflected by the 2nd reflection surface 16b and is observed by an eyepiece lens 11 of the left eye viewing system. When the prism 16 is moved here to align its optical axis B to the optical axis of an objective lens 5, both luminous fluxes R, L are partly totally reflected by the face 16a and are conducted to the right eye viewing system. The fluxes are also partly transmitted therethrough, are totally reflected by the 2nd reflection surface 16b and are conducted to the left eye viewing system, whereby flat observation is accomplished.