A connection state estimating method includes: acquiring a network distance between respective devices in a communication network; assuming branch point candidates at which three devices extracted from the network are connected together; calculating the network distance between each of the assumed branch point candidates and each of the three devices; classifying all the branch point candidates assumed to be connected with one of the devices into groups on the basis of the network distance between the one device and each branch point candidate; calculating representative values of the network distances between each device and branch point candidates assumed to be connected with the device; and selecting a device from the devices assumed to be connected with one of the branch point candidates and the representative value relating to the selected device.