次し第だいに

adverb

gradually; little by little; step by step

Describes a slow, steady change over time. Often used with verbs of change like なる, 変わる, 増える, 減る, etc.

• 空そらが次し第だいに暗くらくなってきた。

  The sky gradually grew dark.

• 彼かれの健けん康こう