To provide a constant vacuum slide throttle valve type carburetor having a simple constitution, a reduced manufacturing cost, a slide throttle valve movement swift and excellent in acceleration performance, and a high air suction efficiency.

An atmospheric pressure chamber 13 and a negative pressure chamber 14 are partitioned by a diaphragm 5 between a body 6 and an upper cover 3. A cup- shaped slide throttle valve 26 is supported in a cylindrical part 6b crossing perpendicularly with an intake passage 33 of the body 6. A cup-shaped negative pressure actuating valve 15 connected to the diaphragm 5 is inserted into the slide throttle valve 26. The stroke of the negative pressure actuating valve 15 is smaller than that of the slide throttle valve 26. Accordingly, only slide throttle valve 26 is actuated in a small opening range, and the slide throttle valve 26 ad the negative pressure actuating valve 15 are integrally actuated in an opening range exceeding the small opening range. A lower end of a negative pressure pipe 18 extending from a bottom wall 15b of the negative pressure actuating valve 15 pierces through a bottom wall 26b of the slide throttle valve 26 and is connected to a detent disk 19. A spring 4 is interposed between an upper cover 3 and the negative pressure actuating valve 15, and a spring 25 interposed between the bottom wall 26b of the slide throttle valve 26 and a detent disk 19.