I am a Canadian permanent resident and living in Canada. I am originally from Afghanistan and would like to visit my girlfriend in Japan. I know Canadian passport holder can simply enter into Japan without a visa and as a permanent resident of Canada can I also enter Japan without a visa for 30 days?
I am afraid NO, as an Afghan national and Canadian permanent resident you need a proper visitor's visa. Answer is from official Consulate-General of Japan in Vancouver:
I am a permanent resident of Canada. Do I need a tourist visa?
It depends on your nationality or the passport you hold. If you hold a passport of a country with a reciprocal visa exemption arrangement with Japan, you do not need a visa to enter Japan as a tourist. Otherwise, you need one. More on Temporary Visitor's Visas.
Unfortunately Afghanistan is not listed in those countries who are listed as a visa exempted country. Source: Ministry of Foreign Affairs Japan. Therefore as an Canadian permenant resident and Afghan national you will need a proper Japan temporary visitor's visa.
Moreover, details regarding temporary visitors visa can be obtained from the following website:
Temporary Visitor's Visa Consulate-General of Japan in Vancouver.