Identity-based (ID-based) cryptosystems eliminate the need for validity checking of the certificates and the need for registering for a certificate before getting the public key. These two features are desirable especially for the efficiency and the real spontaneity of ring signature, where a user can anonymously sign a message on behalf of a group of spontaneously conscripted users including the actual signer. In this paper, we propose a novel construction of ID-based ring signature which only needs two pairing computations for any group size. The proposed scheme is proven to be existential unforgeable against adaptive chosen message-and-identity attack under the random oracle model, using the forking lemma for generic ring signature schemes. We also consider its extension to support the general access structure.