Thanks snakeair for your useful reply. I have read that Adsense post before, but it was still useful now.
Thanks baccifinejewelry for your reply. Even I'm thinking in such a way that if Google allows Adsense placement in a blog post why shouldn't it allows in a forum too?
Thanks turf for your feedback. I think Google will ban me only from that website since it's not my main account if it's against Adsense TOS. Am I right? But even I don't want to get banned from that forum since I found Google is the only best legal way to earn for a normal internet user like me.
There is another reason that we must be very careful with all the first forum post since forums topics can be created by anyone. But blog post can be only created by admin or must be moderated for guest posting. In forum, Some topic created by some forum user may contain contents that may mislead users like this "Visit the following link to get it for free". If adsense is below that it is against adsense TOS. So we must be careful too. That's why I have made rules that all posts made by registered users other than Admins, moderators and other special users must be accepted by a moderator before it appears to public. I think if we follow this tip, all can place Adsense within forum post just like blog post. Am I right?
I'm waiting for more responds from other webmasters. Thanks in advance..