Black outline on button

[jerryberry]

Hi!
When I try to create button for my game, black outlines appear.


The circle at the top is just an empty Game Object with Sprite Renderer. At the bottom is the button made with NGUI.
Atlas' shader is Unlit/Transparent Colored.

Do you know how to remove these outlines?

[ArenMook]

Import the atlas texture with "Alpha is Transparency" checked.