The Diameter Protocol is a AAA protocol that solves the problems of previous AAA protocols such as RADIUS. Diameter EAP Application is one of Diameter applications that supports authentication using EAP. In this research, we designed a new state machine for Diameter EAP Server and implemented the first open-source Diameter EAP Application, called DiamEAP. It is available under BSD-like license. DiamEAP is designed to be extensible so that any new EAP method can be implemented separately as a shared library called EAP method plug-in. Newly added EAP methods are supported without modifications to the DiamEAP implementation. In addition, DiamEAP provides a cryptography library that any EAP method plug-in can use. As one of EAP methods, we implemented EAP-TLS on DiamEAP. We also present our implementation of a Diameter EAP Server and evaluated the basic performance. As a result, DiamEAP's processing time for 20 sessions/second is about 11.9 ms and a duration for authenticating a user is around 45.3 ms. DiamEAP is supported by the AAA working group of the WIDE Project.