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

Here is Why You Need An Adapter

The outlet and plug type used in Equatorial Guinea are C and E 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

Equatorial Guinea

In Equatorial Guinea, outlets of type C and E are in use.

Type C

Type E