It is an astronomical observatory in Silla Period through which they observed the moving of the heavenly body. It is situated near the Royal tomb of King Naemul, and a 9.17m high large stone building.

Liquor bottle-shaped cylinder stands on a platform that plays a role of pedestal, and the Chinese character 井-shaped top is put on the highest part. They made the cylinder by piling 27 steps of fan-shaped stones, and the outer face is trimmed smooth. At the upper part, long ends of stone material that is geared into inside part of 井 shape are extended to the outside. This shape can be seen at the 19th-20th steps and 25th-26th steps, and so it seems that a ladder was easy to put on those steps in the inside. The inside wall face is not even because of protruding endings. Around the opening towards the southeast, the lower part is filled with rubbles, and the upper part to the top is open and hollow. The eastern half of the uppermost step is blocked with a stone slab. A Chinese character 井-shaped stones are put in 2 stories at the highest part, making square. It is guessed that there might have been some facilities for observation.

The record of an ancient book says, 'People can climb it up through the middle.' It seems that one put a ladder on its outside and went into the inside through a window, and then climbed up to the top by ladder to observe the heaven. Once a researcher represented that the top might have been very convenient to sit, stand or lie on.

We can see that the astronomy was deeply related to the agriculture in that it could decide the farming time according to the move of the heaven. And it was also deeply related to the politics in that the horoscope was considered to be important since they could foretell the good and bad luck of a nation according to the result of observing the heaven in the ancient times. Accordingly, the astronomy became a great interest of a nation at the early times, and as a result, it is natural to have built Cheomseongdae (astronomical observatory). The astronomical observatory is estimated to have been built in the period of Queen Seondeok (632∼647) in Silla Period. It is presently a little tilted to the northeast, but remains almost as it used to be. It is highly valued as the oldest astronomical observatory in Asia. And, as a precious cultural property, it shows the development level of the technology at that time.