PLT OF C172 ON BAY TOUR ENTERS CLASS B AIRSPACE BY ONE HALF MI WITHOUT CLRNC. CTLR BRINGS IT TO HIS ATTN AND HE CORRECTS.

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ACFT IS A C172A. I REQUESTED A 'BAY TOUR' DEP FROM THE OAK TWR WITH A FINAL DEST OF THE HALF MOON BAY ARPT ON THE PACIFIC OCEAN SIDE OF SAN MATEO COUNTY. I RECEIVED CLRNC TO FLY 'THE (BAY TOUR) AND REMAIN CLR OF CLASS B AIRSPACE.' I CLBED TO 1900 FT AND MAINTAINED THAT ALT TO STAY BELOW CLASS B AIRSPACE BTWN THE 7-10 NM RINGS OF THE SFO VOR PARALLELING THE PACIFIC OCEAN SHORELINE. ON THE APPROX 270 DEG RADIAL OF SFO VOR; I DRIFTED INSIDE THE 7 NM RING BY .5 NM AND WAS ADVISED BY THE BAY TRACON ON 135.1 THAT MY ALT AT THAT DISTANCE SHOULD BE 1500 FT. I IMMEDIATELY DSNDED TO 1400 FT. I HAD THE VFR TERMINAL CHART IN MY LAP AND THOUGHT I WAS IN COMPLIANCE BUT DIDN'T NOTICE THE 1/2 DRIFT THAT BROUGHT ME INSIDE THE 7 MI RING. I INCORRECTLY ASSUMED FLYING JUST OFFSHORE WOULD KEEP ME CLR OF THE CLASS B AIRSPACE. IT DID NOT! I HAD FLOWN THIS RTE MANY TIMES BEFORE; BUT ALWAYS AT 2500 FT OUTSIDE OF THE 10 MI RING. ON THIS DAY; THE SCATTERED LAYER WAS AT 2000 FT AND I STAYED CLOSER AND LOWER THAN USUAL AND DIDN'T NOTICE THE 6.5 DME INDICATION UNTIL I WAS ASKED FOR MY ALT. STUPID MISTAKE! LULLED INTO COMPLACENCY BY FORMER ASSUMPTIONS!