The Moon interacts with the incident solar wind plasma in various ways, and most of these interactions are accompanied by variations of the interplanetary magnetic field (IMF) around the Moon. Here we first report decreases of the IMF strength observed at 100？km altitude on the lunar dayside and over the polar region, comparing upstream solar wind data from ACE with Kaguya (SELENE) data. We note that the magnetic field decreases are observed above non-magnetized regions or very weakly magnetized regions. In one event the IMF is weakened in the dayside northern hemisphere when the IMF is roughly anti-parallel to the solar wind flow. We estimate that the decrease in the magnetic pressure can be partly compensated by the thermal pressure of the back-scattered solar wind protons, which suggests that the magnetic field decrease is interpreted as diamagnetic effect by the back-scattered protons. In another event an IMF decrease is continuously detected from the northern polar region to the dayside mid-latitude region, which is not fully explained by the thermal pressure of the back-scattered protons. We also discuss the diamagnetic current system in the upstream (fore-moon) solar wind region formed by the back-scattered protons.