Bear with me if I did something wrong. Long story short, I rented a rather cheap old apartment in Japan and the main rooms didn't come equipped with light bulbs. Can anyone help me identify what type of light bulb should I attach on this?
Qualifier: I live in Japan and have seen tons of those sockets.
The majority of Japanese rental apartments do not have ceiling fixtures - tenants bring their own. What you have there is a standard twist-lock ceiling socket. It is designed to hold up lightweight fixtures all by itself, heavier ones hang from the nearby hook.
The somewhat rusty grill beside the outlet will limit your choices somewhat, but a brief visit to any home center or electronics shop will produce a large selection well under Y10,000. The large dish-shaped LED fixtures will not fit on account of the grill, just about anything else is fine.
Keywords you can search for to find something nearby:
- Joyful Honda
- Bic Camera
- Yamada Denki
- Kojima Denki
IMHO this is a somewhat low-quality answer just a cut above link-only (which is why it was posted as a comment), and based entirely on "choose the right keywords for a search about an item I know nothing about directly." But since at least two people have requested it as an answer, here it is. I encourage anyone who actually has direct experience of these or who has some in their home to answer.
Appears to be #15-18 here: plugsocketmuseum "Not grounded twist lock socket and plug for mounting on a ceiling. A (lightweight) lamp or pendant connector plug can be attached to the cable. The plug has to be rotated 35º to detach (16). Images 17 and 18 show the components separately."
I note that you seem to have a hook for a potentially heavier pendant fixture beside it.