The present invention provides a connected sensor system which can be easily used by the user and which can give an external input to each sensor unit without making the wiring complicated, and also provides a network unit and a sensor unit used for the connected sensor system. A cyclic communication memory of a network unit is arranged with an external input request region, and an external input signal is stored to a corresponding memory in a host control apparatus. By performing cyclic communication between the network unit and the host control apparatus, the external input signal is stored to the cyclic communication memory of the network unit. Accordingly, the external input signal is transmitted to each sensor unit connected to the network unit, and each sensor unit executes the external input function.