We have tested endurance of Kapton foil for beam irradiation, which is used as an exit window of in-air PIXE system. kaptoon foil is strong enough to withstand the pressure differential, thin enough to transmit the proton beam with minimal energy loss and does not contain heavier elements. However, little is known about the relation between its ifetime unitl breakdown and beam parameters such as beam current, beam diameter and energy lass. Endurance test were made by an in-air PIXE system at Tohoku university. Irradiation was performed by proton beam with 2 or 3 MeV until the foil was broken for various beam parameters. The total charge until breakdown of the Kapton foil was anayzed in consideratioin of beam current, diameter and energy loss. Total charge until breakdown decrease with increase in the beam current density in case of the beam current density higher than 100 nA/mm~2. The lifetimne of the Kapton foil can be estimated longer than 2 days for the beamn current density of 10 na/mm~2 (7nA in 1 mm beam diameter). It is sufficient for the in-air PIXE analysis.
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