The MWD (measuring while drilling) system, which transmits information such as temperature, pressure, direction, and inclination at the bottom of the hole to the surface of the earth while drilling, is used and is underdeveloped as a higher temperature version, but it was hoped that it would be more efficient in the oil fields and the geothermal fields as a supporting tool in drilling. The transmission from MWD is realized by the pulse interval modulation, which generates pulsed pressure waves and uses the interval of pressure waves to send physical information. This paper shows that our calculation results can accurately simulate the mud pulse in drill strings comparing them with experiments carried out in the laboratory and that, due to the loss of propagation depending upon well depths, the behavior of mud pulse signals apparently change. It is concluded that the simulation program is reliable for this system, and that the mud signal is useful from a 500- to 3000-m depth borehole.