1. An abnormal sign detection apparatus comprising: a data acquisition portion configured to acquire string data made up of a plurality of monitoring items from an apparatus to be monitored at predetermined or arbitrary time intervals; a unit space generation portion including a data temporary storage configured to temporarily store each acquired string data; a data calculation portion configured to calculate an average and variation for each of the monitoring items by using each string data stored in the data temporary storage; and an information calculation portion configured to calculate correlation information indicating a correlation between the monitoring items by using each string data stored in the data temporary storage; a normalization portion configured to normalize the string data acquired by the data acquisition portion using the average and the variation of each monitoring item; a distance calculation portion configured to calculate a distance from the correlation information for normalized string data by carrying out a computation using the normalized string data and the correlation information; and an abnormal sign decision portion configured to decide whether or not there is an abnormal sign in the apparatus to be monitored depending on whether or not calculated distance falls within a confidence interval set in advance for a certain probability distribution, wherein the information calculation portion in the unit space generation unit calculates an inverse matrix of a correlation coefficient matrix among the monitoring items as the correlation information, the distance calculation portion calculates Mahalanobis' distance by carrying out a computation using the normalized string data and the inverse matrix of the correlation coefficient matrix calculates a Euclid distance by carrying out a computation using the normalized string data and a unit matrix corresponding in number to the monitoring items, and calculates the distance by dividing the Mahalanobis' distance by the Euclid distance.