Cooling unit, heat insulating structure, and substrate processing apparatus

There is provided a cooling unit, comprising: an intake pipe provided for each of a plurality of zones and configured to supply a gas for cooling a reaction tube; a control valve provided in the intake pipe and configured to adjust a flow rate of the gas; a buffer part configured to temporarily store the gas supplied from the intake pipe; and openings provided so as to blow the gas stored in the buffer part toward the reaction tube, wherein the flow rate of the gas introduced into the intake pipe is set according to vertical length ratios of the zones such that the flow rate and a flow velocity of the gas injected from the openings toward the reaction tube are adjusted by opening and closing the control valve.