Cannot Program a Custom STM32 Board
Thank you for your great comments. Because of them, I have been confident that my design is error-free. That's why I focused my attention to search for errors in the hardware. And I found the error! This is a soldering error.

I found this error after following this instruction: "Use a multimeter and test all of the connections for shorts or lack of connection. Remember to touch the pins/pads/whatever really gently, because if you press the probe hard, you may "fix" the connection temporarily (for example you push the pin of IC to the trace on PCB and the test is OK, but as soon as you let go the pin bends up again)." The original post is here.