A virtual measuring device and a method for measuring the deposition thickness of amorphous silicon being deposited on a substrate is disclosed, where the method of measuring the deposition thickness of amorphous silicon includes predicting and adapting operations. In the predicting operation, during a process of depositing the amorphous silicon to a substrate, the deposition thickness is predicted by multiplying a predicted deposition speed to a deposition time by using a prediction model expressing a relationship between a deposition speed and a plurality of process factors that are correlated with the deposition speed obtained from the deposition thickness and the deposition time, and the predicted deposition thickness is compared with the measured deposition thickness, so that the relationship between the plurality of process factors and the deposition speed in the prediction model is compensated according to the comparison difference.