A processing furnace for performing prescribed heat treatment on plural substrates, a boat for carrying the plural substrates that are laid one over another in the boat into and out of the processing furnace, a substrate detecting sensor for detecting the plural substrates laid one over another in the boat by changing a relative position of the substrate detecting sensor with respect to each of the plural substrates, and a control section for registering reference positions of the plural substrates and an allowable range of positional deviations from the reference positions of the plural substrates are provided. The control section receives pieces of position information of the plural substrates measured by the substrate detecting sensor, compares a maximum value of pieces of position information of substrates with an average thereof and compares a minimum value of pieces of position information of substrates with an average thereof if the pieces of position information of the plural substrates are out of the allowable range of positional deviations from the reference positions of the plural substrates, and judges that deviation has occurred in a boat stop position if differences determined by the respective comparisons are within the allowable range of deviations of the substrates.