Spot on Dslr Viewfinder but not on images?

I was recently taking pictures with my dslr (Nikon D3100) and noticed that there was a mark (spot) on my viewfinder next to the far left focal point. I took shots anyway and reviewed them on my computer and noticed that there weren't any marks on any of the images. Any advice?

Clean your viewfinder and/or mirror (although any spots on the mirror would probably be out of focus).

A lens pen would be good for this.

You can buy one especially made for rectangular surfaces such as LCD screens and viewfinders. This version has a triangular tip rather than a round one.

If it bothers you that much, have your camera cleaned at the service center. The dirt is either in the viewfinder or the reflex mirror. If not, you can wait a little while longer until the amount of dirt really gets annoying.

Here's a warning about cleaning the reflex mirror. In most instances, attempting to clean the mirror makes it more dirty. The surface is so delicate, tiny amounts of oil from your fingertips smudges on it. Then there's the issue about scratches. Many so-called cleaners easily does so on that mirror.