Is it plagiarism to reuse your own full-text material from a previous conference in a new conference? I am a reviewer at a conference. An author submitted a paper, and I searched the   author's previous papers. I found they have one paper with a very similar title accepted  to a conference in 2012. I downloaded the paper, and about 1.5 pages of the 6 page document is exactly the same as the 2012 paper (all words and everything else.) So is that plagiarism?

There is no reference to their own 2012 work. Without paying attention to the rest of paper? Should I drop that or not?

Update: 
Yes the paper was accepted and indexed on IEEE web site. and I download the paper from IEEE web site.
Yeah, copying 25% of a prior paper without attribution is definitely self-plagiarism.  Report it to the program chairs of the conference, and let them sort out how to manage the problem from there.

Additional information, for any coming from different publication cultures: electrical engineering / computer science conferences are serious publications that take originality very seriously, and IEEE policy on the matter is quite explicit.