CAPT OF A B737-500 OVERSHOT INITIAL CLB LEVELOFF ALT DUE TO BEING DISTR BY THE LNDG GEAR EXTENDING WHEN THE FO MISSED THE GEAR LEVER POS OF 'OFF.'

Narrative

CLRED VIA ORD 1 DEP WHICH IS TO CLB TO AND MAINTAIN 5000 FT MSL. CLBING THROUGH 4500 FT MSL AT 800-1000 FPM RATE OF CLB; THE FO MOVED THE LNDG GEAR LEVER TO 'OFF' PER THE CLB SOP'S TO COMPLY WITH THE CLB CHKLIST. HE INADVERTENTLY HIT THE GEAR DOWN POS AND THE GEAR LOWERED CAUSING A LOUD AIR NOISE AND GEAR UNSAFE LIGHTS. I REACHED FOR THE LEVER AND RAISED THE GEAR AS I SEEMED TO RECOGNIZE THE PROB SOONER. DURING THIS TIME; I EXCEEDED THE 5000 FT CLRNC BY 300-400 FT. DSNDED IMMEDIATELY TO 5000 FT WITH GEAR NOW BACK UP. FO WAS FAIRLY NEW TO AIRLINE OPS AND HAD NEVER SEEN THIS PROB -- TRAINING FOR THIS IS NOT DONE. SINCE I WAS HAND FLYING THE ACFT (A B737-500) I SHOULD HAVE ORDERED 'GEAR UP' RATHER THAN TO DO IT MYSELF. WE WERE BELOW GEAR SPDS AND SOME DELAY WOULD NOT HAVE BEEN A PROB. AT THE TIME; QUICKNESS SEEMED IMPORTANT. THE SURPRISE AND NOISE AT THAT POINT INFLT WAS A FACTOR. DEP CTL WAS NOTIFIED IMMEDIATELY OF OUR DEV AND CLRED US TO 11000 FT. WE THEN LEFT 5000 FT FOR 11000 FT. NO OTHER ACFT WERE SEEN AND I AM NOT AWARE OF A SEPARATION LOSS OCCURRENCE. LESSON LEARNED WAS TO SOLICIT HELP FROM FO RATHER THAN TO TRY TO DO IT ALL MYSELF.