To provide a reliable diagnosing method allowing total inspection without any contact in a manufacturing process of a precast concrete product.

After heating an objective precast concrete product, in a temperature distribution at a time after a short lapse of time before completion of cooling, a part showing a maximum mean temperature and a part showing a minimum mean temperature within an area to be extracted in the precast concrete product are used as reference points, and a temperature difference change with the lapse of time between the reference points and the maximum temperature part and the minimum temperature part is represented in the form of a temperature characteristic curve. In diagnosis, the temperature characteristic curve obtained on the basis of a thermal image signal based on the detection carried out on the measurement objective precast concrete product by means of a thermal infrared sensor is compared with that of the mean temperature shown by a normal precast concrete product, and either of a reinforcement arrangement condition and a cover thickness is diagnosed.