In recent years, various types of applications of electromagnetic techniques for microwave thermal therapy have been developed. Among them, minimally invasive microwave thermal therapies for cancer are of great interest. They are interstitial microwave hyperthermia and microwave coagulation therapy. In this paper, we describe the characteristics of thin coaxial antennas for those therapies. In order to obtain a large heating region, we use an array applicator, which is composed of four coaxial-slot antennas. We analyze the heating characteristics of the array applicator by numerical simulation.