A sample injection device for sample collection and thermal desorption includes: a sample collection structure; a piston type adsorber having an adsorption cavity communicating with the sample collection structure; a piston cylinder defining a piston chamber accommodating the adsorber and communicating with the adsorption cavity; a thermal desorption chamber communicating with the adsorption cavity and the piston chamber; and a pump configured to pump a sample diffused in an ambient gas into the adsorption cavity through the sample collection structure and the piston chamber; the adsorber is movable between a sample collecting position where the adsorption cavity is outside the thermal desorption chamber and adsorbs the sample collected by the sample collection structure and a sample desorbing position where the adsorption cavity is inside the thermal desorption chamber so that the adsorbed sample is thermally desorbed in the thermal desorption chamber.