A fuel valve includes a cylindrical flow adjusting member having a cylindrical surface formed with two circumferentially extending V-shaped grooves for liquid fuel and air, respectively. The fuel valve further includes liquid fuel and air supply pipes having O-rings at their respective discharge ports which are kept in abutment with the cylindrical surface such that when the flow rate adjusting member is rotated to a position where the V-shaped grooves extend across and protrude from the respective O-rings, the discharge ports of the supply pipes communicate with the respective V-shaped grooves, so that liquid fuel and air flow. The depths, widths and positions of the respective V-shaped grooves are determined corresponding to an igniting position (preheating step), normal burning positions (high heat to low heat), and a discharge position such that the flow rates of liquid fuel and air can be adjusted in synchronization with each other.