An apparatus for monitoring quality of a picture being transmitted through a picture transmission path, comprising: characteristic value extraction means (4, 5) for extracting the characteristic values of picture quality at a point on a transmission side of the picture transmission path and another point on a reception side of 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 (4, 5) comprises, for each of said points, block division means for dividing an input picture into blocks each having an arbitrary size, orthogonal transformation means (12, 12a) for subjecting the blocks to an orthogonal transformation, PN sequence multiplication means (31, 31a) for multiplying orthogonal transformation coefficients by a sequence having +1 and -1 at random, inverse orthogonal transformation means (32, 32a) for subjecting the PN multiplied coefficients to an inverse orthogonal transformation, and pixel extraction means (33, 33a) for extracting an arbitrary number of pixels from the pixels obtained by the inverse orthogonal transformation, whereby a same number of pixels at a same plurality of positions is extracted by the pixel extraction means for each said points; and the MSE assessing means comprises square calculation means for, based on said extracted pixels supplied from each of said points, determining a square value of the difference between corresponding pixel values, addition means for adding an arbitrary number of such square values, and picture quality deterioration assessment means for assessing the deterioration of picture quality from the added value obtained by the addition means. characteristic value extraction means (4, 5) for extracting the characteristic values of picture quality at a point on a transmission side of the picture transmission path and another point on a reception side of 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 (4, 5) comprises, for each of said points, block division means for dividing an input picture into blocks each having an arbitrary size, orthogonal transformation means (12, 12a) for subjecting the blocks to an orthogonal transformation, PN sequence multiplication means (31, 31a) for multiplying orthogonal transformation coefficients by a sequence having +1 and -1 at random, inverse orthogonal transformation means (32, 32a) for subjecting the PN multiplied coefficients to an inverse orthogonal transformation, and pixel extraction means (33, 33a) for extracting an arbitrary number of pixels from the pixels obtained by the inverse orthogonal transformation, whereby a same number of pixels at a same plurality of positions is extracted by the pixel extraction means for each said points; and the MSE assessing means comprises square calculation means for, based on said extracted pixels supplied from each of said points, determining a square value of the difference between corresponding pixel values, addition means for adding an arbitrary number of such square values, and picture quality deterioration assessment means for assessing the deterioration of picture quality from the added value obtained by the addition means. the characteristic value extraction means (4, 5) comprises, for each of said points, block division means for dividing an input picture into blocks each having an arbitrary size, orthogonal transformation means (12, 12a) for subjecting the blocks to an orthogonal transformation, PN sequence multiplication means (31, 31a) for multiplying orthogonal transformation coefficients by a sequence having +1 and -1 at random, inverse orthogonal transformation means (32, 32a) for subjecting the PN multiplied coefficients to an inverse orthogonal transformation, and pixel extraction means (33, 33a) for extracting an arbitrary number of pixels from the pixels obtained by the inverse orthogonal transformation, whereby a same number of pixels at a same plurality of positions is extracted by the pixel extraction means for each said points; and the MSE assessing means comprises square calculation means for, based on said extracted pixels supplied from each of said points, determining a square value of the difference between corresponding pixel values, addition means for adding an arbitrary number of such square values, and picture quality deterioration assessment means for assessing the deterioration of picture quality from the added value obtained by the addition means.