BEFORE STARTING THE FLT, THE ACFT WAS LOADED WITH FULL FUEL, 5 ADULT PASSENGERS & APRX 45 LBS OF BAGGAGE. THE ELEVATION & TEMP AT THE ARPT WERE 3606 FT & 93 DEG. THE PLT SAID HE DID NOT CALCULATE THE WEIGHT & BALANCE OR DENSITY ALTITUDE LIMITS FOR THIS FLT BECAUSE HE HAD PREVIOUSLY FLOWN THIS ACFT UNDER SIMILAR CONDITIONS & HAD NOT ENCOUNTERED ANY PROBLEMS. THE PLT INITIATED A NO FLAP TAKEOFF, AS WAS REQUIRED. HE REPORTED THAT THE ACFT WAS SLOW TO ACCELERATE DURING THE TAKEOFF ROLL. AFTER LIFT-OFF, HE RETRACTED THE LANDING GEAR, BUT THE ACFT WOULD NOT CLIMB & BEGAN TO DECELERATE. TO THE PLT, THERE APPEARED TO BE INSUFFICIENT RWY REMAINING FOR A WHEEL LANDING, SO HE ELECTED TO LAND WITH THE GEAR UP. AN INVESTIGATION REVEALED THE ACFT WAS ABOUT 400 LBS OVER ITS CERTIFICATED MAX GROSS WEIGHT LIMIT & THE DENSITY ALT WAS 6528 FT. ALSO, THE PLT STATED THAT THE ENGS & ALL SYSTEMS APPEARED TO BE WORKING WELL. THE WIND WAS VARIABLE & GUSTING FROM CALM TO 5 KTS. JUST BEFORE STARTING TAKEOFF, THE WIND WAS REPORTED FROM 070 DEG AT 5 KTS.