You Do Need power plug adapter while traveling from Guinea to Japan.

Here is Why You Need An Adapter

The outlet and plug type used in Japan are A and B 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

Japan

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

Type A

Type B