PURPOSE: To suppress the probability of excluding a meter reading value from an allowable range to a low level even when the allowable range for meter reading values is highly accurately narrowed at the time of automatically diagnozing the validity of a meter reading value in an automatic meter reading system.

CONSTITUTION: A 36 calculating part 143 finds out M±3σ (M is a mean value and σ is a standard deviation) for lamp meter reading values for the past three months in each week. In this case, the M±3σ is corrected so as to cancel the influence of an isolation time upon the using quantity of lamps based upon isolation time data applied from an isolation time data applying part 144. An appointed month means value calculating part 145 calculates the means value of the meter reading value of that month in each week. A comparing part 146 compares whether the means value is included within the range of M±3σ or not, and when included in the range, outputs a message indicating the validity of the meter reading value of the month.