Who is military advisor to secretary of defense?

Technically, the SecDef does not have any specific position which is designated as his "military advisor". As the SecDef, he is the immediate superior to the entire US Military, and will generally consult directly with the head of any department or Command when information is needed. While the staff of the SecDef often contains a significant number of military personnel, they are not in a specific advisory role.

The President of the United States currently relies on the Chairman of the Joint Chiefs of Staff to be the primary military advisor and liaison.