You Do Need power plug adapter while traveling from Guinea 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 Guinea are C, F and K. This is how they look like.

Guinea

In Guinea, outlets of type C, F and K are in use.

Type C

Type F

Type K

Niger

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

Type C

Type D

Type E

Type F