A GA pilot landed at a wrong airport which was 5 miles away and with the same orientation as his CTAF destination.

Narrative

We decided to take a trip to Minneapolis and then continue our trip to OSH. I obtained a weather briefing at the airport 1 hour before takeoff; weather was no issue. While crossing Lake Michigan; we obtained flight following with MKG half way across the lake. About 30 miles east of Minneapolis; we contacted Minneapolis Approach and got flight following to SGS. Staying clear of Class B Airspace we were told to change frequencies to SGS Unicom and squawk 1200. As we approached the airport we announced our position for landing. Another aircraft replied that he would be clear of the runway before we got there. Announcing; 'short final; Runway 34;' we could see an aircraft taxing off the runway. He said; 'clear of active runway;' and we responded; 'Thank you.' Everything seemed normal; all visuals were clear for landing. Approximately 100 FT AGL I started to question the runway because it was wider with many black skid marks. I thought they had expanded the runway and were allowing larger aircraft to land. At that point I realized something was wrong; and the safest thing to do was to continue the landing. After I landed I saw the Tower and knew I was at the wrong airport. I taxied to a safe spot off the runway. As we gathered our thoughts; we examined the chart and knew immediately what I had done wrong. We looked up the frequency and did the normal procedure to depart Class D Airspace. After radio contact was made; the Tower Controller gave me a phone number to call when I landed at SGS. I called and discussed the matter with the Supervisor at STP; he said he would write up a report. These two airports are approximately 5 miles apart from each other; their runways are only 20 degrees different. Everything seemed normal when I was making my approach to what U thought was SGS but was actually STP. I thought I was going into an uncontrolled airport so I did not obtain radio contact with STP prior to entering their airspace.