A gas supply device for supplying gas to filling equipment that is configured to fill the gas in a device provided with a tank, said gas supply device comprising: a compressor that is configured to compress the gas; an accumulator disposed downstream of the compressor and configured to supply the gas to an outflow end to which the filling equipment is connectable; and a gas flow passage that connects the compressor, the accumulator, and the outflow end, characterized in that the gas flow passage includes: a main line (161, 162) connecting the compressor to the outflow end; a lead-in line connected with the main line (161, 162) for leading the gas from the compressor into the accumulator; a lead-out line for leading out the gas from the accumulator to the outflow end; a lead-in side valve provided in the lead-in line; and a lead-out side valve provided in the lead-out line, the gas supply device further comprises a control device that is configured to control opening and closing of the lead-in side valve and the lead-out side valve to selectively perform: a series differential pressure filling operation of allowing the gas to flow from the compressor to the accumulator and from the accumulator to the outflow end by simultaneously bringing the lead-in side valve and the lead-out side valve into an open state, and a direct filling operation of allowing the gas to flow from the compressor to the outflow end directly by simultaneously bringing the lead-in side valve and the lead-out side valve into a closed state. a compressor that is configured to compress the gas; an accumulator disposed downstream of the compressor and configured to supply the gas to an outflow end to which the filling equipment is connectable; and a gas flow passage that connects the compressor, the accumulator, and the outflow end, characterized in that the gas flow passage includes: a main line (161, 162) connecting the compressor to the outflow end; a lead-in line connected with the main line (161, 162) for leading the gas from the compressor into the accumulator; a lead-out line for leading out the gas from the accumulator to the outflow end; a lead-in side valve provided in the lead-in line; and a lead-out side valve provided in the lead-out line, a main line (161, 162) connecting the compressor to the outflow end; a lead-in line connected with the main line (161, 162) for leading the gas from the compressor into the accumulator; a lead-out line for leading out the gas from the accumulator to the outflow end; a lead-in side valve provided in the lead-in line; and a lead-out side valve provided in the lead-out line, the gas supply device further comprises a control device that is configured to control opening and closing of the lead-in side valve and the lead-out side valve to selectively perform: a series differential pressure filling operation of allowing the gas to flow from the compressor to the accumulator and from the accumulator to the outflow end by simultaneously bringing the lead-in side valve and the lead-out side valve into an open state, and a direct filling operation of allowing the gas to flow from the compressor to the outflow end directly by simultaneously bringing the lead-in side valve and the lead-out side valve into a closed state.