An automatic pH adjustment device comprising: a container that is configured to accommodate a liquid sample having a pH indicator added thereto, color of the pH indicator is configured to change in accordance with pH; an adjusting liquid bottle that is configured to store aqua ammonia or carbonated water used as a pH adjusting liquid; a nebulizer that is provided with an injection port disposed to face a sample liquid surface in the container and configured to inject from the injection port the pH adjusting liquid stored in the adjusting liquid bottle and atomized by a carrier gas injected from the nebulizer; a multi-way valve that is disposed between the nebulizer and the adjusting liquid bottle and is configured to perform switching between a liquid injection state in which the nebulizer is allowed to communicate with the adjusting liquid bottle to inject the pH adjusting liquid from the injection port, and an air injection state in which the nebulizer is allowed to communicate with outer air to inject air from the injection port; a detecting device that is configured to detect an intensity of light radiated from the outside of the container and transmitted through the container; and a control device that is configured to switch the communication state of the multi-way valve on the basis of the intensity of light detected by the detecting device, the detecting device is configured to detect an intensity I a of light of a specific wavelength absorbed by the pH indicator if the sample is adjusted to a target pH and an intensity I r of light of a reference wavelength not absorbed by the pH indicator if the sample is adjusted to the target pH, characterized in that the control device is configured to calculate an intensity ratio R i obtained by dividing the intensity I a by the intensity I r, and is configured to switch the communication state of the multi-way valve between the liquid injection state and the air injection state by a comparison of the calculated intensity ratio R i with a target intensity ratio R 0 set in accordance with the target pH. a container that is configured to accommodate a liquid sample having a pH indicator added thereto, color of the pH indicator is configured to change in accordance with pH; an adjusting liquid bottle that is configured to store aqua ammonia or carbonated water used as a pH adjusting liquid; a nebulizer that is provided with an injection port disposed to face a sample liquid surface in the container and configured to inject from the injection port the pH adjusting liquid stored in the adjusting liquid bottle and atomized by a carrier gas injected from the nebulizer; a multi-way valve that is disposed between the nebulizer and the adjusting liquid bottle and is configured to perform switching between a liquid injection state in which the nebulizer is allowed to communicate with the adjusting liquid bottle to inject the pH adjusting liquid from the injection port, and an air injection state in which the nebulizer is allowed to communicate with outer air to inject air from the injection port; a detecting device that is configured to detect an intensity of light radiated from the outside of the container and transmitted through the container; and a control device that is configured to switch the communication state of the multi-way valve on the basis of the intensity of light detected by the detecting device, the detecting device is configured to detect an intensity I a of light of a specific wavelength absorbed by the pH indicator if the sample is adjusted to a target pH and an intensity I r of light of a reference wavelength not absorbed by the pH indicator if the sample is adjusted to the target pH, characterized in that the control device is configured to calculate an intensity ratio R i obtained by dividing the intensity I a by the intensity I r, and is configured to switch the communication state of the multi-way valve between the liquid injection state and the air injection state by a comparison of the calculated intensity ratio R i with a target intensity ratio R 0 set in accordance with the target pH.