The meet-in-the-middle attack can be used for forging signatures on mixed-type digital signature schemes, and takes less time than an exhaustive attack. This paper formulates a meet-in-the-middle attack on mixed-type digital signature schemes, shows the necessary conditions for success, and discusses the relationships between computational and space complexities as well as success probability during the attack. We also analyze an optimal strategy for forgers to apply this attack, pointing out that an intermediate value of 64 bit length is not secure for any mixed-type digital signature scheme. Finally, we show how to design secure mixed-type digital signature schemes.