Abstract: A skin condition observation apparatus having a sebum amount measuring device adapted for bringing a sebum sampling surface into contact with skins and optically measuring the amount of deposited sebum, wherein the sebum amount measuring device comprises an orthogonal prism having two reflection surfaces orthogonal with each other in which one reflection surface is exposed as the sebum sampling surface, and a light emitting element for illuminating a light to one of the reflection surfaces and a photoreceiving element for detecting the intensity of a reflection light reflected on the two reflection surfaces and returned therefrom are located being opposed to the hypotenuse surface at the back of the orthogonal prism, with their optical axes being in parallel with each other, whereby the sebum amount measuring device of the skin condition observation apparatus can be manufactured with no requirement for glass fabrication at high accuracy, troublesome and delicate alignment for optical axes with respect to the