To ensure flame stability, even in a transient load range by reducing the flow sectional area of an air inflow slit formed in the tangential direction with respect to the partial cone of a burner in the flowing direction.

A premixing burner has two hollow partial cones 101, 102. The cones 101, 102 are deviated from each other and entangled with each other. The central axes or longitudinal symmetrical axes 101b, 102b of the cones 101, 102 are deviated from each other. Air inflow slits 119, 120 of a tangential direction are opened at the mirror symmetrical disposition at both sides. Combustion air 115 is fed to a conical hollow chamber 114 via the slits 119, 120. The flow sectional areas of the slits 119, 120 are reduced in the flowing direction. Thus, the concentration of liquid fuel is reduced continuously by the air 115 fed in the axial direction for optimal mixing.