You Do Need power plug adapter while traveling from Nauru to Niger.

Here is Why You Need An Adapter

The outlet and plug type used in Niger are C, D, E and F and in Nauru is I. This is how they look like.

Nauru

In Nauru, outlets of type I are in use.

Type I

Niger

In Niger, outlets of type C, D, E and F are in use.

Type C

Type D

Type E

Type F