It's free to receive an SMS while you're roaming in Japan, with your SMARTY SIM card @Jackylam.
Japan is in SMARTY Mobile's "Roaming 3" band for roaming rates, which are given on page 19 of the online price guide (SMARTY-Price-Guide.pdf).
You can't use your plan's allowances while roaming in Japan, so to make/receive calls, send texts, and use roaming mobile data, you'll need to buy an add-on for your plan.
