Exposing method and apparatus

A method of exposing a long iron plate from both sides thereof, wherein said iron plate has a photosensitive material coated on both sides thereof, is held between a first mask plate provided with a first masking pattern on the inner side thereof and a second mask plate provided with a second masking pattern on the inner side thereof, comprising the steps of:

• simultaneously image-sensing first and second alignment patterns formed at those areas on the inner sides of the first and second mask plates, respectively where the iron plate is not held by the first and second mask plates so as to obtain an image signal said first and second alignment patterns being in a predetermined positional relation when the first and second mask plates are correctly aligned with each other, said first alignment pattern comprising a pair of circular marks arranged in a direction parallel to the long axis of the iron plate and said second alignment pattern comprising a pair of circular marks arranged in the the a direction perpendicular to the long axis of the iron plate;
• processing the image signal to determine the positionally mis-registered amount of the first and second alignment patterns from the predetermined positional relation;
• exposing both sides of the iron plate to photoradiation through the first and second mask plates;
• moving one of the first and second mask plates in a direction along the iron plate to correct the positionally mis-registered amount of the alignment pattern determined by the processing of the image signal;
• moving the iron plate by a certain length;
• and
• repeating the previous steps.