1. An automatic analyzer for analyzing a blood sample using a plurality of reagents, comprising: a container for containing a blood sample; a plurality of containers for containing a plurality of reagents; a dispensing section for dispensing the sample and the plurality of reagents; a reaction section including a substrate having a plurality of reaction containers, each container for allowing the blood sample to react with one of the plurality of reagents therein; a photometry section for capturing an image of the inside of each of the reaction containers; a control section; an analysis section coupled to the control section, the analysis section for analyzing the sample on the basis of whether or not reactions have occurred inside the reaction containers: and, a storage section coupled to the control section and comprising a control program, the control program for implementing processing executed by the automatic analyzer, the control section configured to: analyze a plurality of images of the inside of the plurality of reaction containers captured by the photometry section, wherein the analysis comprises calculating a measurement result comprising a numerical value of a photometric parameter of each of the images in accordance with the luminance of the sample, wherein the numerical value of the photometric parameter is compared with a threshold value to determine whether an agglutination reaction is positive or negative, and analyzing characteristic information of the sample; wherein when no agglutination reaction has occurred in the reaction containers being defined as negative, calculate a difference between a maximum value and a minimum value of the photometric parameter for the different reagents used to analyze the same sample when the agglutination reactions for the different reagents are negative, determine whether or not the measurement result is valid by comparing the calculated difference with a predetermined threshold of the photometric parameter, and add a validity result of the determination to characteristic information of the corresponding sample.