A software system has dynamic adaptability if it can adapt itself to dynamically changing runtime environments. As open-ended distributed systems and mobile computing systems have spread widely, the need for software systems with dynamic adaptability increases. We propose a software model with dynamic adaptability called DAS and its description language LEAD++. In the DAS model, the basic mechanism for dynamic adaptability is based on adaptable procedures. An adaptable procedure is a variant of generic procedure (function) whose methods are selected depending on the state of its runtime environment. Furthermore, control mechanisms of adaptable procedures --- including method selection strategies --- are realized using adaptable procedures. Such reflective architecture enables us to write a dynamically adaptable software system in highly flexible and extensible way. LEAD++ is an object-oriented reflective language that provides adaptable procedures and their control mechanisms. We are currently implementing a prototype of LEAD++ as a pre-processor of Java. Using LEAD++, we can systematically describe dynamically adaptable mobile objects, etc.