on the face of it is an adverb meaning "from appearances alone". It carries one recorded sense.
from appearances alone
“irrigation often produces bumper crops from apparently desert land”
“the child is seemingly healthy but the doctor is concerned”
“had been ostensibly frank as to his purpose while really concealing it”
“on the face of it the problem seems minor”
Definitions, synonyms and semantic relations for on the face of it come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.