A particle monitoring method of monitoring particles included in an exhaust gas from a depressurized processing vessel 12 includes counting the particles included in the exhaust gas from the depressurized processing vessel 12 while cleaning an inside of the depressurized processing vessel 12 by a particle monitor 18; creating a histogram showing a time and the number of particles from a result of the counting of the particles; extracting, from the histogram, a first feature amount indicating a correlation between a mode of the number of the particles and a particle counting period; and extracting, from the histogram, a second feature amount indicating a correlation between the particle counting period and a distribution tendency of the particles during the particle counting period.