CSS div doesn't fill top and left

I am trying to make a fixed header by putting my header content inside a div with a fixed position. Below is the CSS code for the div:

#page_header {
    background-color: white;
    height: auto;
    width: 100%;
    position: fixed;
    overflow: auto;
}

and this is the effect of my div, (screenshot of the left top corner):

You can clearly see that there is a space between the div(white part) and the page, while it is not the case on the right side, how should I get rid of this space?

Thanks.