You Do Need power plug adapter while traveling from Japan 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 Japan are A and B. This is how they look like.

Japan

In Japan, outlets of type A and B are in use.

Type A

Type B

Niger

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

Type C

Type D

Type E

Type F