An object is to provide a plasma processing device capable of properly monitoring a state of plasma discharge and detecting a precursor to abnormal discharge, and a method of monitoring the state of discharge in the plasma processing device. In analysis processing for monitoring executed by detecting, by a signal analyzing portion 30, a signal of a potential change induced in a discharge detection sensor 23 in response to a change in plasma discharge of the inside of a processing chamber and recorded in a signal recorder 20, based on a counter value N3 obtained by detecting a signal of abnormal discharge (first arc discharge) generated between an electrode portion and an object to be processed by a first detector 33 and a counter value N4 obtained by detecting a signal of micro-arc discharge (second arc discharge) generated by deposition of a foreign substance in the processing chamber by a second detector 35, an abnormal discharge determining portion 39 calculates a difference (N3−N4) and compares the difference (N3−N4) with a threshold value a2 for determination and determines whether there is a possibility of generation of abnormal discharge in the processing chamber.