1. A method for monitoring a semiconductor manufacturing process, the method comprising: a learning step for collecting SEM images of patterns formed on a learning sample and information on cross-sectional shapes of the patterns corresponding to the SEM images, learning a relationship between a quantity of a first image characteristic calculated from the collected SEM images and cross-sectional shapes of the patterns corresponding to the collected SEM images, and selecting an estimation engine among plural estimation engines stored in a memory which fits for estimating a cross-sectional shape of a pattern from the calculated characteristic of the SEM image; an estimation step for estimating a cross-sectional shape of a pattern to be measured, using a second image characteristic of a SEM image of a pattern to be measured that is formed on a sample, by applying the second image characteristic to the selected estimation engine; a reliability calculation step for calculating reliability of the estimated crosssectional shape of the pattern to be measured by using both the second image characteristic, and a relationship between the quantity of the first image characteristic and cross-sectional shapes of the patterns of the learning sample; an evaluation step for evaluating the calculated reliability of the estimated cross-sectional shape of the pattern to be measured by comparing an index value of the calculated reliability with a reference value; and a displaying step for displaying the estimated cross-sectional shape of the pattern to be measured with a data of the calculated reliability when the index value of the calculated reliability is greater than the reference value, wherein the reliability calculation step is performed without information regarding the true cross-sectional shape of the pattern to be measured.