Not all ATMS around the World accept JCB cards, however the number of ATMs that do is increasing
JCB cards, whilst not being as widespread as Visa / MasterCard / American Express, are still able to be used at any ATM in the world that features a JCB, JCB Cards or Cirrus logo. ATMs that display this are available in countries such as Taiwan, Hong Kong, South Korea, China, Australia, Russia, UAE, US, UK, Italy, France, Germany, Spain, Austria, Denmark (and more), however not all ATMs/vendors in a country accept the JCB card and your mileage may vary. It appears also, that the cards are accepted by certain banks (and their ATMs) rather than non-specific ATMs.
Currently worldwide coverage is increasing (according to the increasing number of JCB monthly press releases).
The JCB website itself does not manage a list of ATMs (at least not on
either their Global or Japanese websites), however the periodic press
releases seem to mention which banks provide JCB card service.
OCT 14, 2016: Bank Saint Petersburg to Start Accepting JCB Cards;
"...The bank network includes 7,100 POS terminals and 700 ATMs located not only in Saint Petersburg but also throughout the Leningrad region, in Moscow and Kaliningrad."
SEP 26,2016: Bank of Bhutan to start accepting JCB Card;
"With this launch, JCB cards are accepted at more than 480 locations and it covers 90% of POS terminals and ATMs in the market."
If you search specifically for the country you are looking to travel to, you should find a press release stating the JCB acceptance.
Global Website Search Page can be accessed HERE.