You Do Need power plug adapter while traveling from Niger to Guinea-Bissau.

Here is Why You Need An Adapter

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

Niger

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

Type C

Type D

Type E

Type F

Guinea-Bissau

In Guinea-Bissau, outlets of type C are in use.

Type C