To provide a water-based ball point pen for good writing performance which makes it hard for the ball to be eroded by ageing and keeps excellent smoothness during writing.

The water-based ball point pen employs a cemented carbide ball comprised of tungsten carbide and a metal bonding material for its leading edge for writing, wherein the metal bonding material contains a cobalt and/or a nickel at 1 to 20 wt.% of the total volume of the cemented carbide ball, and furthermore, incorporates a water-based ink composition containing colorant, water, 2,5-Dimercapto-1,3,4-thiadiazole, and a surfactant selected from the salts of N-acylamino acid and N-acylmethyltaurine.