1. An apparatus for monitoring quality of picture in transmission for monitoring the quality of a picture being transmitted through a picture transmission path connected in series to a plurality of transmission apparatuses, comprising: characteristic value extraction means for extracting the characteristic values of picture quality at a plurality of points on the picture transmission path; and MSE assessment means for assessing an MSE (mean square error) from the characteristic values extracted by the characteristic value extraction means, wherein: the characteristic value extraction means comprises block division means for dividing an input picture into blocks each having an arbitrary size, orthogonal transformation means for subjecting the pictures in the blocks to an orthogonal transformation, and frequency component value extraction means for extracting an arbitrary frequency component value of the orthogonal transformation; and the MSE assessment means comprises square calculation means for selecting frequency component values at two arbitrary points from the frequency component values supplied from the plurality of points and determining the square value of the difference between corresponding frequency component values, addition means for adding an arbitrary number of the square values, and picture quality deterioration assessment means for assessing the deterioration of picture quality from the added value obtained by the addition means.