We propose the notion of plaintext awareness in the two-key setting, called PATK.
We also prove that
if a public-key encryption scheme is secure in the sense of PATK, 
then it is also secure in the sense of IK-CCA.
Since it looks much easier to prove that a public-key encryption scheme is secure 
in the sense of PATK than to prove directly that it is secure in the sense of IK-CCA, 
the notion of PATK is useful to prove the anonymity property of public-key encryption schemes.

We also propose the first generic conversion for the anonymity, that is, 
we prove that the public-key encryption scheme derived from the Fujisaki-Okamoto conversion scheme, 
where the basic public-key encryption scheme is secure in the sense of IK-CPA, 
is secure in the sense of IK-CCA in the random oracle model.