Sprite rendering problem

Hi! I've got some problems with rendering my sprite. It's a walls sprite for my game. The .png image looks good and in the Unity preview window there's not problem too. But when I run the game, I see the contours of the sprite square. See the images below:

Image in preview (I mark the contour of sprite square by red color):

And the image when the game started:

It's hard to see the contour in second image, but if you make the image bigger you can see that there're the Unity rendering bug! How to fix it? Did you know? Sorry for my bad English, I hope you know what I mean.