What would be a rent per month?

Student house offers as low as JPY25,000 per month.

What would be a initial costr?

Student house offeres as low as JPY75,000 per month.

Can I check in on the same day?

Yes.
Some apartment takes a few days for a landlaw's aproval.

Do I need to have a guarantor?

Basically, student house offers apartmens or share house which does not require guarantoss. However, some of them require guarantor system.

What's 'SHIKIKIN'?

'SHIKIKIN' is a safety deposit.
Generally, 'SHIKIKIN' is equivalent to 2-3 month rent. However, Student House offeres the apartment with less 'SHIKIKIN'.
Basically 'SHIKIKIN' is refundable on checking out.

What's 'REIKIN'?

'REIKIN' is a give away money and is none refundable. Student House offers apartment withour 'REIKIN'.

What's Garantor system?

Guarantor system is to take part of a guarantor, for a person who does not have one.
The charge is from 50% -100% of the rent.

Is furnished apartment common in Japan?

No.
Generally, apartment is not furnished.
No curtain, No Electric appliances, No furniture.

What would be a charge for a furnished apartment?

Furnished apartment is rather expensive compare to general apartments. Aproximately, 50%-70 higher.

Will I save money by staying at furnished apartment?

If your stay is short, you can dsfinitely save money by using furnished apartment.
Not only avoiding purchasing furnitures, initial cost is much lower in furnished apartment.