noun which may take the genitive case particle 'no'
outward; superficial; external; apparent
Describes how something looks from the outside, often implying that the inner reality may be different. Used attributively with の.
外見上の問題はないように見える。
There seems to be no outward problem.
外見上の変化はほとんどなかった。
There was almost no apparent change.
表面上 also means 'on the surface' or 'superficially', but is more common and can be used adverbially with に.
Compound of 外見 (がいけん, 'outward appearance') + 上 (じょう, suffix meaning 'from the standpoint of').