Searching JSTL document and found the answer

Looking for the detail about JSTL c:set because of JSTL helps us but... not clear for me... - Shammerism and found following site.

java - I can pass a variable from a JSP scriptlet to JSTL but not from JSTL to a JSP scriptlet without an error - Stack Overflow

According to this site, the expression like ${resp} is same as pageContext.getAttribute("test");.
The JSTL c:set can accept the argument whose name is scope, and it's default is page scope. So, just do following

<c:set var="HOGE" value="SOMETHING"></c:set>

is equivalent as

pageContext.setAttribute("HOGE", "SOMETHING");

Now I can travel between the world of JSTL and script-let at will, maybe...

By the way, Oracle document of this is https://docs.oracle.com/javaee/5/jstl/1.1/docs/tlddocs/c/set.html. There is no information I would like to know about.