The influence of ion stimulated gas desorption (ISD) from the ion-extraction plate of a residual gas analyzer (RGA) ion source on partial pressure measurement was evaluated, using a modified RGA. ISD from the extraction plate occurs due to bombardment with ionized gas molecules, and is thought to be closely related to physical sputtering and chemical reactions on the plate surface. Our experimental results indicate that partial pressure detection limits deteriorate due to an ISD signal increase, which in turn is dependent on total pressure, the species and kinetic energy of the ion, and the temperature of the ion-extraction plate. Where the ion-extraction plate is heated to 250 degreesC and the dominant gas species is massive or reactive, ISD signals account for an amount on the order of 100 ppm of partial pressure signal intensity. This ISD problem can affect the accuracy of partial pressure measurement in all pressure ranges. However, there are several methods to reduce ISD. Among the possible methods, a simple modification of the ion source to restrict the ion-bombarded area would diminish ISD the most effectively.