Kubo, T. JACoW Geneva, Switzerland 978-3-95450-178-6 10.18429/JACoW-SRF2015-MOPB009 English 90-94 MOPB009 target cavity interface experiment framework Contribution to a conference proceedings 2015 2015-12 http://dx.doi.org/10.18429/JACoW-SRF2015-MOPB009 http://srf2015.vrws.de/papers/mopb009.pdf Recent findings that cooling conditions affect an amount of trapped magnetic flux attract much attention as a way to achieve a high-Q0 by SRF cavity*,**,***. Q0~2*10¹¹ has already been achieved by the full flux expulsion****. While much experimental studies have been conducted, not much theoretical progress followed on it. In this paper, I introduce a simple model that can explain how trapped fluxoids are expelled in cooling process.