Costco is widely known in UK but it is not common for people living in mainland Europe. Instead of Costco, Metro/Makro is popular in mainland Europe which Costco does not operate.
The differences between Costco and Metro/Makro are simple. Costco charges annual membership fee but Metro/Makro not. On the other hand, Metro/Makro is not open for general public in most of the countries.
There are 26 Costco warehouses in Japan. These warehouses are not located in downtown of big cities but in the suburb. If you are in Japan and going to stay a long period, Costco is a must place to visit.