Question about acquiring japanese nationality

Hi im sachie my father is a japanese and my mother is a filipina my father acknowledge me and My name is already included in his koseki i just want to know if i can still acquire the japanese nationality even if im already 22 years old now? thankyou

No; you must be under 20. https://www.ph.emb-japan.go.jp/files/000369188.pdf

What makes you think you are not Japanese?

OP was born out of wedlock, and probably outside Japan. In this case, a child is not automatically a Japanese citizen at birth.

"my father is a japanese"
"My name is already included in his koseki"

you are a Japanese.
you have to choose one nationality, because Japan only allows one nationality for one person.

Wow. So we range from :
1) you are Japanese
2) you are likely not Japanese
3) you are not Japanese and will never be

Sounds like we need to get a vote done to decide the favorite option;-)

Or consult with someone who actually knows. Embassy or immigration lawyer might be a good place to start.

In order for him to be Japanese his father had to register him in his koseki AND he or his parents needed to report his intention to acquire Japanese nationality before 20. It's not clear if he did the latter but probably not since he's asking this question.

Get a copy of your koseki and ask the Japanese embassy if you're not sure.

I don't know exactly when the law changed, but for a 22yo person I believe the act of a parent to place them into the Koseki is in itself a declaration by the parent that the child is Japanese. The better question might be, What action (if any) caused a loss of nationality.

I didn't see in the OP where it stated born out of wedlock, and don't know if there is an impact for a 22yo who is in the koseki.

Agree, consultation with a lawyer may be needed.

my father acknowledge me

If the parents are married, no acknowledgement is required, as the mother's husband is automatically recognised as the father (even if DNA testing shows he's not the biological father!).

even if DNA testing shows he's not the biological father!
False.

"My name is already included in his koseki"
if it means that "the acknowledgement" is recorded in his koseki, you are not a Japanese yet.
(you don't have your own koseki, your name is in supplement of your father's koseki.)
you had to take the next paperwork until 20 years old.

"even if DNA testing shows he's not the biological father!"
I think that is correct.
when the legal father finds that he is not a biological father, he has to go to a court and get a judgement that there is no father-child relationship between the two. otherwise, they are still legally the father and the child.

generally speaking, "acknowledgement" itself is rare in Japan, because it makes troubles in future.
(getting a child without marriage is rare in Japan, when they are well educated.)