1. A control method of a malfunction condition judgment apparatus that judges a malfunction condition of an observed object based on a change of observed values observed from the observation object, comprising the steps of: acquiring time series data for values of each of a plurality of variables that has a plurality of observed values observed from the observation object as a value; calculating, with respect to each of the variables, a statistic defining a probability density function of that variable at a point of time (T 1 ), by adding into a value where the statistic at the prior time to T 1 is subtracted by a value obtained by multiplying a forgetting rate set corresponding to the variable at the prior time to said point of time, a value where a value of the variable at T 1 is multiplied by said forgetting rate; calculating, with respect to each of the variables, dissimilarity showing an extent of variation between the statistic calculated for that variable and a statistic of a criterial probability density function predetermined corresponding to that variable; pick up, out of the plurality of variables, a variable in which the calculated dissimilarity is larger than a predetermined reference value, as the variable by which a malfunction condition of the observed object is detected; and setting a forgetting rate with respect to each of the variables in a manner that, in a case where an updating frequency at which that variable is updated in a unit time is higher, the forgetting rate is set lower than in a case where the updating frequency is lower.