In a pretreatment instrument and a pretreatment method of saliva, used for identification and quantitation of Streptococcus mutans in saliva by the immunochromatographic method utilizing an antigen-antibody reaction, the instrument includes a swab and a mixing container for saliva and a treatment liquid, the swab having a stick and a soft synthetic resin-made sponge capable of absorbing a predetermined amount or more of saliva, and the mixing container being made of a transparent or translucent soft synthetic resin and comprising a bag-like portion formed integrally with and continuously to a constricted portion as the end of a tapering introduction portion having an opening thoroughly larger than the sponge, wherein the constricted portion and the bag-like portion have elasticity such that the sponge can be squashed by a finger pressure in a state that the sponge is inserted therein; the constricted portion has a width such that a pressure can be applied by fingers and has a shape such that when the sponge is taken out in a squashed state by the finger pressure, a minimum amount necessary for the pretreatment or more of the saliva can be squeezed out.