Disclosed is a particle density measuring probe for measuring the density of atoms or molecules in a plasma atmosphere by absorption spectroscopy. The probe has a cylindrical light guiding member provided in the plasma atmosphere. At the front end of the light guiding member, there is provided a reflection plate for reflecting light that has propagated through the cylindrical light guiding member. Behind the reflection plate, in a cross section perpendicular to the longitudinal direction of the light guiding member, a part devoid of a portion of wall surface is provided by a predetermined length in the longitudinal direction. A plasma introducing portion allows mutual contact between light passing through this part devoid of a portion of wall surface and atoms or molecules in the plasma atmosphere. The probe has a main body that guides light in an axial direction by total reflection by a side wall, and that is located behind the plasma introducing portion.