Is it really difficult for fereigners to rent a house? How to proceed?

Let's just say it's not easy. Here's a website which lists what you need to rent a house. https://blog.gaijinpot.com/rent-an-apartment-in-japan/

And below are some foreigner friendly agents/websites:
Real Estate Japan: https://www.realestate.co.jp/ 
Rent a House in Japan: https://www.japan-experience.com/rent-a-house-in-japan
Housing Japan https://housingjapan.com/rent/
Apartments in Gaijinpot: https://apartments.gaijinpot.com/en/rent/listing/
Japan Home Finder: http://www.japanhomefinder.com/

Wish you the best of luck!

Hi Rodrigo, 

Thank you for sharing all those websites! It is really helpful!

I'm going to check it out before make my journey to Japan!