We propose a new aggregate signature scheme from bilinear pairings. It requires constant pairing operations in the verification and the size of aggregate signature is independent of the number of signers. In the public key infrastructure (PKI) setting, it is the first aggregate signature scheme with these properties. Hence, it is very practical in the environments with computational power limited. We prove it secure in the random oracle model assuming the intractability of the computational Diffie-Hellman.