Monitoring and diagnosing device including: a classification information storage section; frequency information storage section: a first data classifier section reading out reference classification information from the classification information storage section, comparing operational data, detected by a plurality of sensors and inputted in time sequence, with the reference classification information to classify the operational data, and then generating operational data classification information; a frequency comparator section compiling the operational data classification information, generating operational data frequency information by adding, to the operational data classification information, appearance frequency information for each classification of operational data, reading out reference frequency information from the frequency information storage section, and then generating operational data frequency comparison information by comparing operational data frequency information with the reference frequency information; and an abnormality diagnosing section performing an abnormality diagnosis upon the working machine by use of the operational data classification information and operational data frequency comparison information.