The main barriers are cost and discrimination - although you will also need to have a guarantor. In the Tokyo area, they have a system called reikin , a non-refundable deposit usually called 'key money' in English. In other areas, the reikin system is not usually used but refundable deposits are higher. And the term 'refundable' is open to interpretation, too. Usually, a third or so is kept by the landlord for redecoration or replacing tatami mats or whatever.
Discrimination tends to be pretty blatant.
A couple of guys might want to share a place to cut down on cost - an ax murderer has a better chance of moving in, as long as he's Japanese. There are also restrictions on pets, children, students etc. Rents vary greatly depending on location, size, age of the building and the floor the apartment is on. A possible source of confusion is the Japanese words for the two types of apartments - apaato and mansion . Apaato means a wooden apartment building, often old and with little or no sound or heat insulation.