To provide an underwater heavy metal removing agent which can sophisticatedly adsorb/remove the ion of a heavy metal such as lead in the water, particularly in tap water, can also efficiently remove residual chlorine and trihalomethane and can give delicious and healthful water having good balance between calcium ion and magnesium ion.

This underwater heavy metal-removing agent contains a synthetic zeolite and an activated carbon by the weight ratio of 2:98 to 50:50. The synthetic zeolite to be used is obtained by replacing ≥10 mol% of the total amount of exchangeable cations with magnesium ion and ≥60 mol% with magnesium ion and calcium ion.