I registered my site a month ago, and appeared in google search result of focused keyword. The ranking is not so bad, my site placed in 4th~5th page when google results have 9~10 pages. But after 1 day the ranking falled 4~5 pages. So, my page appeared in 9th page. In some computers, there is no my page in SERP. Of course, I didn't take any Black-hat SEO. Is my site penalized by google? Can my site increase ranking again?
The ranking will change according to their algorithm. It may change each day. If you content is useful and is not copied from else where, then your ranking will increase eventually.