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

Here is Why You Need An Adapter

The outlet and plug type used in Namibia are D and M 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

Namibia

In Namibia, outlets of type D and M are in use.

Type D

Type M