A network construction support system adapted to propose a layout of one or more units of construction assisting tools to be installed in an apparatus network system having a plurality of communication apparatuses connected by multi-drop, each of the construction assisting tools being an apparatus used for narrowing down a range that involves a cause location of a fault when the fault occurs in the apparatus network system, the network construction support system comprising: a layout data generation unit to generate one or more pieces of layout data indicating a layout of the construction assisting tools to be installed in the apparatus network system according to the layout indicated in the layout data; an index value calculation unit to calculate a countermeasure cost and an estimated risk amount on the basis of inherent information data, for each layout data, said inherent information data indicating information inherent to a case that is aimed at construction of the apparatus network system and specifically including skill level data, loss data and management system data, the countermeasure cost increasing as a number of units of construction assisting tools increases, the estimated risk amount decreasing as the number of units of construction assisting tools increases and as a narrowed range by each construction assisting tool narrows; a layout data judging unit to judge an appropriateness of the layout data for the apparatus network system, for each layout data, on a basis of the countermeasure cost, the estimated risk amount, and an allowable risk amount which is allowed for the apparatus network system; and a result output unit to output layout data judged to be appropriate for the apparatus network system. each of the construction assisting tools being an apparatus used for narrowing down a range that involves a cause location of a fault when the fault occurs in the apparatus network system, the network construction support system comprising: a layout data generation unit to generate one or more pieces of layout data indicating a layout of the construction assisting tools to be installed in the apparatus network system according to the layout indicated in the layout data; an index value calculation unit to calculate a countermeasure cost and an estimated risk amount on the basis of inherent information data, for each layout data, said inherent information data indicating information inherent to a case that is aimed at construction of the apparatus network system and specifically including skill level data, loss data and management system data, the countermeasure cost increasing as a number of units of construction assisting tools increases, the estimated risk amount decreasing as the number of units of construction assisting tools increases and as a narrowed range by each construction assisting tool narrows; a layout data judging unit to judge an appropriateness of the layout data for the apparatus network system, for each layout data, on a basis of the countermeasure cost, the estimated risk amount, and an allowable risk amount which is allowed for the apparatus network system; and a result output unit to output layout data judged to be appropriate for the apparatus network system. a layout data generation unit to generate one or more pieces of layout data indicating a layout of the construction assisting tools to be installed in the apparatus network system according to the layout indicated in the layout data; an index value calculation unit to calculate a countermeasure cost and an estimated risk amount on the basis of inherent information data, for each layout data, said inherent information data indicating information inherent to a case that is aimed at construction of the apparatus network system and specifically including skill level data, loss data and management system data, the countermeasure cost increasing as a number of units of construction assisting tools increases, the estimated risk amount decreasing as the number of units of construction assisting tools increases and as a narrowed range by each construction assisting tool narrows; a layout data judging unit to judge an appropriateness of the layout data for the apparatus network system, for each layout data, on a basis of the countermeasure cost, the estimated risk amount, and an allowable risk amount which is allowed for the apparatus network system; and a result output unit to output layout data judged to be appropriate for the apparatus network system.