A method for setting transmission timing of messages by allocating transmission timing to a plurality of messages with predetermined various transmission cycles, the method comprising: grouping the messages by transmission cycle into transmission cycle subgroups in each of which the maximum number of messages that can be included is determined by a predetermined criterion for grouping by transmission cycle; arbitrarily setting an order of the grouped transmission cycle subgroups and arbitrarily setting an order of messages included in each of the transmission cycle subgroups; and (i) setting a transmission grid corresponding to the number of the grouped transmission cycle subgroups; (ii) selecting one message according to the set order of the messages, in the set order of the transmission cycle subgroups, based on the transmission grid; (iii) allocating the selected message; and (iv) repeating the steps of (ii) to (iii) until the allocation of the messages of all the transmission cycle subgroups completes, wherein the predetermined criterion for grouping by transmission cycle is that the maximum number of messages that can be included in a transmission cycle subgroup is determined by dividing the transmission cycle of a message included in the transmission cycle subgroup by the greatest common divisor of transmission cycles of a plurality of messages to be grouped, characterized in that the value of the transmission grid is set so as to satisfy both a first grid value setting condition and a second grid value setting condition; wherein the first grid value setting condition is that the transmission grid value must be set between a grid division minimum value and a grid division maximum value, the grid division minimum value being determined by multiplying the transmission duration of a message by a predetermined group coeffcient, the grid division maximum value being the greatest common divisor of the transmission cycles; and wherein the second grid value setting condition is that the value determined by dividing the greatest common divisor of the transmission cycles by the transmission grid value must be equal to or more than the number of the transmission cycle subgroups. grouping the messages by transmission cycle into transmission cycle subgroups in each of which the maximum number of messages that can be included is determined by a predetermined criterion for grouping by transmission cycle; arbitrarily setting an order of the grouped transmission cycle subgroups and arbitrarily setting an order of messages included in each of the transmission cycle subgroups; and (i) setting a transmission grid corresponding to the number of the grouped transmission cycle subgroups; (ii) selecting one message according to the set order of the messages, in the set order of the transmission cycle subgroups, based on the transmission grid; (iii) allocating the selected message; and (iv) repeating the steps of (ii) to (iii) until the allocation of the messages of all the transmission cycle subgroups completes, wherein the predetermined criterion for grouping by transmission cycle is that the maximum number of messages that can be included in a transmission cycle subgroup is determined by dividing the transmission cycle of a message included in the transmission cycle subgroup by the greatest common divisor of transmission cycles of a plurality of messages to be grouped, characterized in that the value of the transmission grid is set so as to satisfy both a first grid value setting condition and a second grid value setting condition; wherein the first grid value setting condition is that the transmission grid value must be set between a grid division minimum value and a grid division maximum value, the grid division minimum value being determined by multiplying the transmission duration of a message by a predetermined group coeffcient, the grid division maximum value being the greatest common divisor of the transmission cycles; and wherein the second grid value setting condition is that the value determined by dividing the greatest common divisor of the transmission cycles by the transmission grid value must be equal to or more than the number of the transmission cycle subgroups.