There is a connection between mold vibration and surface quality of billets. We have designed measuring system for evaluating correlation between the surface defects and the occurrence of non-linearities in the time-frequency vibrations in mold. For elimination the influence of a warm and humid environment there is a sensorial part inserted into the steel casing. Signals are fed to the input of the 12 b AD converter, which records the data on a computer disk for post-processing. This system includes software that controls the digitization process. This software allows instant preview of the level of the vibration energy in the time-frequency domain, which is calculated using Fast Fourier transform algorithm.