Ohmi, K. Funakoshi, Y. Hirosawa, K. Koiso, H. Morita, A. Ohnishi, Y. Zhou, D. JACoW Publishing Geneva, Switzerland 978-3-95450-216-5 10.18429/JACoW-eeFACT2018-TUOBB01 English 66-70 TUOBB01 luminosity coupling emittance MMI optics Contribution to a conference proceedings 2019 2019-04 https://doi.org/10.18429/JACoW-eeFACT2018-TUOBB01 http://jacow.org/eefact2018/papers/tuobb01.pdf Collision in SuperKEKB phase II commissioning has started in April 2018. Luminosity was lower than the geomterical value even in very low bunch current. Linear x-y coupling at IP caused by skew of QCS was conjectured as error source. x-y coupling correction using skew corrector of QCS resulted in luminosity recover of 2 times. After the QCS skew correction, luminosity is still limited at relatively low bunch current. Nonlinear x-y coupling at IP is conjectured as a source of the luminosity limitation. We discuss effects of linear and nonlinear x-y coupling at IP on the beam-beam performance.