The authors propose an M/G/1 queueing model in which the server is replaced by another one with different service time distribution when the number of customers in the system becomes larger than a threshold value. A random set up time is needed to introduce the second server. The server works until the system becomes empty. The behavior of the customers who receive the first and second kinds of service is analyzed using the theory of finite Markov chain and the decomposition property of the M/G/1 system with server vacations, respectively. Some important performance measures such as the distribution and average of the number of customers in the system and their average time in the system are obtained. The influence of the characteristic of the servers, the condition when and how the server is replaced, and the distribution of the set up time on the performance of the system are made clear. [In Japanese.]
