A network test device for testing operation of a gateway device connecting a first network and a second network, the network test device comprising: first electronic message receiving means connected to the first network for receiving electronic messages traveling on the first network; second electronic message receiving means connected to the second network for receiving electronic messages traveling on the second network; received electronic message storage means storing electronic messages respectively received by the first electronic message receiving means and the second electronic message receiving means; mapping information creating means estimating the correlation between input electronic messages input into the gateway device from the first network and output electronic messages output on the second network from the gateway device, based on multiple electronic messages stored in the received electronic message storage means, and creating mapping information consisting of information obtained as a result of the estimation; an object information database in which properties and property values are correlated in levels for all or a portion of objects managed by the gateway device; test electronic message creating means creating an input electronic message for test based on the mapping information and the object information database; electronic message sending means outputting the input electronic message for test to the first network; and test result determining means determining whether or not the test result is normal by determining whether or not the relationship between the input electronic message for test and one or multiple output electronic messages is correct with reference to the mapping information after output of the input electronic message for test by the electronic message sending means. first electronic message receiving means connected to the first network for receiving electronic messages traveling on the first network; second electronic message receiving means connected to the second network for receiving electronic messages traveling on the second network; received electronic message storage means storing electronic messages respectively received by the first electronic message receiving means and the second electronic message receiving means; mapping information creating means estimating the correlation between input electronic messages input into the gateway device from the first network and output electronic messages output on the second network from the gateway device, based on multiple electronic messages stored in the received electronic message storage means, and creating mapping information consisting of information obtained as a result of the estimation; an object information database in which properties and property values are correlated in levels for all or a portion of objects managed by the gateway device; test electronic message creating means creating an input electronic message for test based on the mapping information and the object information database; electronic message sending means outputting the input electronic message for test to the first network; and test result determining means determining whether or not the test result is normal by determining whether or not the relationship between the input electronic message for test and one or multiple output electronic messages is correct with reference to the mapping information after output of the input electronic message for test by the electronic message sending means.