An operation control method of a refrigeration unit for land transportation in which a plurality of evaporator units (5A, 5B) are parallelly connected into a refrigerant circuit for circulating a refrigerant by a compressor driven by a dedicated drive source, each of the plurality of the evaporator units (5A, 5B) having an evaporator (14A, 14B), and a plurality of different transport temperatures can be respectively created for a plurality of refrigerated sections which are distributedly arranged with the plurality of evaporator units, the method comprising: employing throttle mechanisms whose opening degrees are adjustable at upstream sides of the evaporators, respectively, providing a low pressure sensor which detects the evaporating pressure of the refrigerant from said evaporator units, for said refrigerant circuit; and performing a priority control for lower setpoint temperature which preferentially distributes the refrigerant to the evaporator unit corresponding to the refrigerated section having a lower setpoint temperature, when a plurality of operation requests have been made for a plurality of said evaporator units, during a various temperature cooling operation having different setpoint temperatures for said respective refrigerated sections, wherein the throttle mechanisms are electronic expansion valves (13A, 13B) and in that the priority control comprises controlling the opening degrees of the electronic expansion valves (13A, 13B) to achieve an evaporating pressure which can provide the refrigerating capacity necessary for the refrigerated section having the lower setpoint temperature, and in that said priority control for lower setpoint temperature is selected in a case where the following conditions are all satisfied: a condition where a difference in the setpoint temperature (Tset) between respective refrigerated sections is greater than a predetermined value (a); a condition where an inside temperature (Tair) of the lower setpoint temperature side is lower than the setpoint temperature (Tset) of the higher setpoint temperature side; and a condition where the rate of change in the inside temperature (ΔTair) of the lower setpoint temperature side is smaller than a predetermined value (β). employing throttle mechanisms whose opening degrees are adjustable at upstream sides of the evaporators, respectively, providing a low pressure sensor which detects the evaporating pressure of the refrigerant from said evaporator units, for said refrigerant circuit; and performing a priority control for lower setpoint temperature which preferentially distributes the refrigerant to the evaporator unit corresponding to the refrigerated section having a lower setpoint temperature, when a plurality of operation requests have been made for a plurality of said evaporator units, during a various temperature cooling operation having different setpoint temperatures for said respective refrigerated sections, wherein the throttle mechanisms are electronic expansion valves (13A, 13B) and in that the priority control comprises controlling the opening degrees of the electronic expansion valves (13A, 13B) to achieve an evaporating pressure which can provide the refrigerating capacity necessary for the refrigerated section having the lower setpoint temperature, and in that said priority control for lower setpoint temperature is selected in a case where the following conditions are all satisfied: a condition where a difference in the setpoint temperature (Tset) between respective refrigerated sections is greater than a predetermined value (a); a condition where an inside temperature (Tair) of the lower setpoint temperature side is lower than the setpoint temperature (Tset) of the higher setpoint temperature side; and a condition where the rate of change in the inside temperature (ΔTair) of the lower setpoint temperature side is smaller than a predetermined value (β).