two months’ security deposit

[suprunp]

In order to rent this apartment he had to pay two months’ security deposit: one month’s rent up front, plus a fee equivalent to the second month’s rent. The rent itself wasn’t that high, and the security deposit would be returned when the lease was up, but still, this all came to a hefty sum.
(H. Murakami; 1Q84)

I don't understand what the security deposit consists of. If it is going to be returned when the lease is up then it can't consist of 'one month’s rent up front, plus a fee equivalent to the second month’s rent', can it?
Would you be so kind to clear it up?

Thanks.

[5jj]

You are right.