Today I went into the tunnel with a portable barometer with taking data, to check whether the pressure difference is actual or not.
When I moved from the Hokubu-kaikan to the Atotsu entrance, the pressure decreased by about 10 hPa. (due to the heater in the building?)
When I was in the tunnel, the portable barometer shows lower pressure than the outside about 2 hPa.
However, the difference was smaller than that of the permanently equipped barometers are showing.