Disclosed is a magnetoencephalography meter which includes a plurality of optical pumping magnetometers which are arranged in a helmet shape to cover the head of the subject. Elongated grooves are provided in the margin of one of the barrel halves. The straps are held on the suction tube by screws and are arrow-shaped in the zone of their base limbs so that the lint is unimpeded as it flows past this surface. The one or more statistics are useable to modify which data items are included in the set of data items. Accordingly, as write to a memory cell in an overerased state can evenly be performed, influence by off-leakage is suppressed, and a memory cell having threshold voltage distribution with less variation can be implemented.