Serious; earnest — showing a sincere, sober attitude and taking things (work/study/etc.) seriously.
彼はいつも真面目に仕事をしている。
He always works seriously.
まじめな顔で説明を聞いていた。
She was listening with a serious expression.
毎日まじめに日本語を勉強している。
I study Japanese diligently every day.
Honest; sincere — having integrity, not deceitful, straightforward and trustworthy.
彼女はとても誠実で、まじめな人だ。
She is very sincere and an honest person.
まじめな考え方が評価された。
Her sincere way of thinking was appreciated.
面接ではまじめさをアピールした方がいい。
It's better to emphasize your sincerity in an interview.