In order to process a few thousands of streams in real-time, public clouds seem to be an excellent choice since it supports massively amount of computing resources. A public cloud may be managed by a third party and outside the firewall of the organization, which may make it questionable whether data are managed conffidentially. This paper proposes a scheme that evaluates queries over encrypted data streams. Our scheme is based on CryptDB and its modification. We describe performance issues incurred by our scheme, and propose an approach to reduce the encryption costs and amounts of transmitted data size, and an approach to reduce memory usage by analyzing a plan tree in an SPE. The result of experiments showed that applying AP-1 reduces 90 % memory size, while applying AP-2 reduces 11 % memory size.