Selective reduction of work function on a W(100) field emitter was observed due to co-adsorption of Si and Ti, followed by angular confinement of field emission on the top (100) surface. Si was vacuum deposited on the side surface near the apex of the field emitter by an amount, which is enough for spreading by 1 ML. Then it was flash heated to spread the deposited Si over the apex surface. Ti was vacuum deposited over it from the opposite side and heat treated to spread over. The work function, which is determined from the Fowler-Nordheim plot, reduced to 3.3 eV. It is assumed that the work function of the clean tungsten is 4.5 eV. The experimental evidences suggest that there is a certain Ti/Si ratio for giving the minimum value of the work function.