There are provided a heap memory management method that not only improves processing performance of an application program but also reduces a program development cost and a computer system using the same method. A live object copy process in minor collection of generational garbage collection is implemented through a calculate process of object live ratio, a compare process of object live ratio, and a live object copy process. The calculate process of object live ratio calculates a ratio of a size of a memory allocated to live objects of new objects to a size of a memory allocated to a new object during minor collection. The compare process of object live ratio compares a calculated object live ratio with a predetermined threshold. The live object copy process copies a live object directly to an old-generation heap instead of a new-generation heap when the object live ratio exceeds the threshold as a result of comparison. Further, an application programmer provides means for setting the threshold that decides a copy destination of the live object as a runtime parameter of a program and means for holding the set threshold in a memory.