Device (10, 20, 30) for injecting hydrogen into a flow of natural gas coming from a first natural gas distribution network, characterized in that it comprises: - a gas line configured to be connected to the first distribution network, and through which a natural gas flow passes; - a hydrogen supply line comprising a hydrogen buffer storage tank; - a sensor detecting the flow rate of the natural gas flow passing through the connecting line, providing a signal representative of the flow rate measured; - a means for the regulated injection of the hydrogen stored in the buffer storage tank into the gas passing through the connecting line as a function of the flow rate measured in order to form a gas mixture; - a sensor detecting at least one characteristic of a composition of the gas mixture; - a means for supplying the mixture of natural gas and hydrogen, injected into a second natural gas distribution network; - a control means configured to compare each said characteristic of the composition of the mixture detected to a defined range of values for said characteristic; - a temporary storage volume of the mixture of natural gas and hydrogen; - a valve for supplying the mixture into the second network, characterized in that - the sensor detecting at least one characteristic of a composition detects at least one characteristic of the composition of the mixture stored in the temporary storage volume; and - the valve supplying the mixture into the second network is connected to the control means; - the means for controlling the valve supplying the mixture into the second network is configured to allow the supply to the second network when each characteristic of the composition detected matches the range of values defined for said characteristic; and - the storage volume being configured to retain the stored mixture for a period of time corresponding to an operating period of the sensor of at least one characteristic of the composition of the gas mixture. - a gas line configured to be connected to the first distribution network, and through which a natural gas flow passes; - a hydrogen supply line comprising a hydrogen buffer storage tank; - a sensor detecting the flow rate of the natural gas flow passing through the connecting line, providing a signal representative of the flow rate measured; - a means for the regulated injection of the hydrogen stored in the buffer storage tank into the gas passing through the connecting line as a function of the flow rate measured in order to form a gas mixture; - a sensor detecting at least one characteristic of a composition of the gas mixture; - a means for supplying the mixture of natural gas and hydrogen, injected into a second natural gas distribution network; - a control means configured to compare each said characteristic of the composition of the mixture detected to a defined range of values for said characteristic; - a temporary storage volume of the mixture of natural gas and hydrogen; - a valve for supplying the mixture into the second network, - the sensor detecting at least one characteristic of a composition detects at least one characteristic of the composition of the mixture stored in the temporary storage volume; and - the valve supplying the mixture into the second network is connected to the control means; - the means for controlling the valve supplying the mixture into the second network is configured to allow the supply to the second network when each characteristic of the composition detected matches the range of values defined for said characteristic; and - the storage volume being configured to retain the stored mixture for a period of time corresponding to an operating period of the sensor of at least one characteristic of the composition of the gas mixture.