A plasma density monitor for monitoring a plasma density of surface wave plasma in a chamber accommodating a substrate and performs a plasma process on the substrate. The monitor includes: a monopole antenna installed to extend from a wall of the chamber toward an interior of the chamber and to be perpendicular to an inner wall surface of the chamber, and configured to receive a surface wave; a coaxial line configured to extract a detection value from a signal received by the monopole antenna; a length adjuster configured to adjust a length of the monopole antenna; and a controller configured to control the length adjuster so as to obtain a wavelength of the surface wave and the plasma density of the surface wave plasma from the wavelength of the surface wave.