Can anyone tell me about scalping in Japan?

There is a sold out concert that I want to see, so I was thinking of just going to the stadium on the night of the show to see if I can buy one on the street.

Is it illegal? What happens if you get caught?

Alright,
so as far as I know, it is illegal to buy tickets from scalpers in Japan because to resell tickets or whatever at higher prices than its real value is illegal.
So if you buy it, it means you are helping a "criminal."

But, if you go to the stadium and are asked to pay 5000 yen for a ticket by somebody and she/he bought it at 5000yen on the other day, you may buy the ticket from her/him.
Because she/he resells it but sells for 5000 yen, which is the real value of the ticket.

Sorry for my poor English.
Hope you will be able to get to the concert!