A network system, comprising: a main programmable logic controller, PLC, configured to carry out, in every first cycle, (i) a process of receiving data from a first slave device, (ii) a process of carrying out, in accordance with control sequences determined through a load simulation carried out in advance by a management device, a part of a first process with respect to the data which has been received, (iii) a process of transmitting data corresponding to a remainder of the first process to an auxiliary PLC in accordance with the control sequences determined through the load simulation and receiving a result corresponding to the remainder of the first process from the auxiliary PLC, and (iv) a process of transmitting, to the first slave device, a result corresponding to the first process; the auxiliary PLC, which is configured to transmit, to the main PLC before a period allocated to the first process within the first cycle ends, the result corresponding to the remainder of the first process with respect to the data received from the main PLC; a first network which is an industrial network controlled by the main PLC which is configured to transmit data between the first slave device and the main PLC; a second network which is a high speed/synchronization network and is configured to transmit data between the main PLC and the at least one auxiliary PLC so as to enable synchronization among a cycle of control carried out by the main PLC, a cycle of control carried out by the auxiliary PLC, and a cycle of communication of the first network and a cycle of communication of a third network; and the third network, which is an industrial network controlled by the auxiliary PLC and which is configured to transmit data between a second slave device and the auxiliary PLC, wherein a reference cycle of control by the main PLC of the first network and a reference cycle of control by the auxiliary PLC of the third network being synchronized with each other, wherein the respective reference cycle is a shortest cycle of control of the respective PLC (20A, 20B), and wherein the first cycle having a length equal to a length of the reference cycle of the control by the main PLC of the first network or having a length obtained by multiplying, by a constant number, the length of the reference cycle of the control by the main PLC of the first network. a main programmable logic controller, PLC, configured to carry out, in every first cycle, (i) a process of receiving data from a first slave device, (ii) a process of carrying out, in accordance with control sequences determined through a load simulation carried out in advance by a management device, a part of a first process with respect to the data which has been received, (iii) a process of transmitting data corresponding to a remainder of the first process to an auxiliary PLC in accordance with the control sequences determined through the load simulation and receiving a result corresponding to the remainder of the first process from the auxiliary PLC, and (iv) a process of transmitting, to the first slave device, a result corresponding to the first process; the auxiliary PLC, which is configured to transmit, to the main PLC before a period allocated to the first process within the first cycle ends, the result corresponding to the remainder of the first process with respect to the data received from the main PLC; a first network which is an industrial network controlled by the main PLC which is configured to transmit data between the first slave device and the main PLC; a second network which is a high speed/synchronization network and is configured to transmit data between the main PLC and the at least one auxiliary PLC so as to enable synchronization among a cycle of control carried out by the main PLC, a cycle of control carried out by the auxiliary PLC, and a cycle of communication of the first network and a cycle of communication of a third network; and the third network, which is an industrial network controlled by the auxiliary PLC and which is configured to transmit data between a second slave device and the auxiliary PLC, wherein a reference cycle of control by the main PLC of the first network and a reference cycle of control by the auxiliary PLC of the third network being synchronized with each other, wherein the respective reference cycle is a shortest cycle of control of the respective PLC (20A, 20B), and wherein the first cycle having a length equal to a length of the reference cycle of the control by the main PLC of the first network or having a length obtained by multiplying, by a constant number, the length of the reference cycle of the control by the main PLC of the first network.