rEFInd

I installed Ubuntu Linux on my dyanbook N40, but when I tried to boot it, it displayed "System Reset" and kept rebooting. I had no choice but to try various other distributions. At last, I noticed that the boot options of the Slackware installation USB allowed me to select a boot file other than USB. It can select not only other Linux distribution but also maybe Windows, I guess.

I see... I didn't know there was a way to do that (:-). So, I have established the operation of booting Ubuntu on my Dynabook N40 from Slackware's installation USB by pressing F12 (:-) every time I start up my computer.
I had been using Ubuntu with this operation for a while, but I decided to install rEFInd, thinking that there must be a smarter way nowadays. I found "Install to Storage" option on the rEFInd USB boot screen, so I tried it and ,after all,  Ubuntu booted from Dynabook N40 without F12. The combination of rEFind + Linux is the best for Linux installers who are suffering from UEFI problems. I wish they should make it a standard feature (:-).