Storage Check settings, there is more than 300 MBs of temporary files since last month. I manually cleared and cleared IE cache, but it's not going below than 315 MB till this moment. There's no pending download or installation at this moment. How can I clear this memory?
In the normal operation of the phone, it will require small amounts of space for temporary files, which is what you're seeing.
Although you can clear it via Storage Check, as the phone is still active it's probably using some of those temporary files, so not all can be deleted.
The GDR2 update, which should reach all Lumia Windows Phone 8 devices by the end of September, does help to reduce the number of temporary and 'other' files left behind.
I checked two Lumia's both running GDR2...
- Lumia 820 - 12mb temporary files
- Lumia 920 - 14mb temporary files
There are also various Windows Phone related news sites which have reported that GDR2 does seem to help or correct the issue...