An analytical apparatus with a function of plugging a container with a cap , comprising: a container mounting table configured to hold the container in a container setting position while being plugged; an optical sensor having at least one light source and at least one light detector arranged as a pair between which the container setting position is arranged, wherein said container setting position is arranged so that: - a region where the cap and the container overlap each other is irradiated with light of the light source when the container is placed at the container setting position; and - light emitted from the light source and having passed the container reaches the light detector as transmitted light, both in a situation where the cap is mounted on the container and in a situation where the cap is pressed into the container to complete the plugging, wherein the optical transmittance of the cap is not more than the optical transmittance of the container , and wherein the apparatus is configured to detect: - whether or not the container has been set, - whether or not the cap exists on the container , and - whether or not the cap is pressed into the container to complete the plugging, in accordance with a quantity of transmitted light detected by the light detector , wherein said apparatus is configured to: - prevent said cap member from being transferred onto the container until it has been detected that the container has been set in the container setting position; and - prevent the container from being transferred until it has been detected that the cap is pressed into the container to complete the plugging. a container mounting table configured to hold the container in a container setting position while being plugged; an optical sensor having at least one light source and at least one light detector arranged as a pair between which the container setting position is arranged, wherein said container setting position is arranged so that: - a region where the cap and the container overlap each other is irradiated with light of the light source when the container is placed at the container setting position; and - light emitted from the light source and having passed the container reaches the light detector as transmitted light, - a region where the cap and the container overlap each other is irradiated with light of the light source when the container is placed at the container setting position; and - light emitted from the light source and having passed the container reaches the light detector as transmitted light, both in a situation where the cap is mounted on the container and in a situation where the cap is pressed into the container to complete the plugging, wherein the optical transmittance of the cap is not more than the optical transmittance of the container , and wherein the apparatus is configured to detect: - whether or not the container has been set, - whether or not the cap exists on the container , and - whether or not the cap is pressed into the container to complete the plugging, - whether or not the container has been set, - whether or not the cap exists on the container , and - whether or not the cap is pressed into the container to complete the plugging, in accordance with a quantity of transmitted light detected by the light detector , wherein said apparatus is configured to: - prevent said cap member from being transferred onto the container until it has been detected that the container has been set in the container setting position; and - prevent the container from being transferred until it has been detected that the cap is pressed into the container to complete the plugging. - prevent said cap member from being transferred onto the container until it has been detected that the container has been set in the container setting position; and - prevent the container from being transferred until it has been detected that the cap is pressed into the container to complete the plugging. <EOS>