A connecting device for piping for connecting a pipe to another pipe or to another part for piping, the connecting device comprising: an adapter body having a hollow structure, the adapter body comprising a cap-coupling threaded portion formed in one end thereof, a nipple-coupling threaded portion formed in an opposite end thereof, and a tapered seating portion formed therein such that a width thereof gradually decreases from the nipple-coupling threaded portion toward the cap-coupling threaded portion; a cap comprising a cap through-hole formed therein to allow a pipe to pass therethrough and a cap threaded portion formed so as to correspond to the cap-coupling threaded portion to be threadedly engaged with the adapter body; a body-sealing member disposed in the adapter body so as to contact an outer circumferential surface of a pipe entering the adapter body; a grip ring disposed in the adapter body, the grip ring comprising a plurality of latching pieces configured to be elastically deformed by contact with an outer circumferential surface of a pipe entering the adapter body to fix the pipe to the adapter body; a pipe nipple fixed to a pipe or another part for piping, the pipe nipple comprising a nipple through-hole formed therein to allow a fluid to flow therethrough, a nipple threaded portion formed so as to correspond to the nipple-coupling threaded portion to be threadedly engaged with the adapter body, and a nipple tapered portion protruding in a forward direction of the nipple threaded portion while gradually decreasing in width; and a metal sealing member comprising a sealing member body provided at an outer side thereof with an outer contact surface tapered so as to closely contact the tapered seating portion and provided at an inner side thereof with an inner contact surface tapered so as to closely contact the nipple tapered portion, the metal sealing member being disposed in the adapter body to provide a seal between the adapter and the pipe nipple, wherein the adapter body comprises a protruding portion formed to have a constant width between the nipple-coupling threaded portion and the tapered seating portion, wherein the pipe nipple comprises a nipple cylindrical portion having a cylindrical shape and formed to have a width smaller than a width of the protruding portion between the nipple threaded portion and the nipple tapered portion, and wherein the metal sealing member comprises a plurality of sealing member protrusions protruding from an edge of the sealing member body so as to be located on the protruding portion and to contact an outer surface of the nipple cylindrical portion 153). an adapter body having a hollow structure, the adapter body comprising a cap-coupling threaded portion formed in one end thereof, a nipple-coupling threaded portion formed in an opposite end thereof, and a tapered seating portion formed therein such that a width thereof gradually decreases from the nipple-coupling threaded portion toward the cap-coupling threaded portion; a cap comprising a cap through-hole formed therein to allow a pipe to pass therethrough and a cap threaded portion formed so as to correspond to the cap-coupling threaded portion to be threadedly engaged with the adapter body; a body-sealing member disposed in the adapter body so as to contact an outer circumferential surface of a pipe entering the adapter body; a grip ring disposed in the adapter body, the grip ring comprising a plurality of latching pieces configured to be elastically deformed by contact with an outer circumferential surface of a pipe entering the adapter body to fix the pipe to the adapter body; a pipe nipple fixed to a pipe or another part for piping, the pipe nipple comprising a nipple through-hole formed therein to allow a fluid to flow therethrough, a nipple threaded portion formed so as to correspond to the nipple-coupling threaded portion to be threadedly engaged with the adapter body, and a nipple tapered portion protruding in a forward direction of the nipple threaded portion while gradually decreasing in width; and a metal sealing member comprising a sealing member body provided at an outer side thereof with an outer contact surface tapered so as to closely contact the tapered seating portion and provided at an inner side thereof with an inner contact surface tapered so as to closely contact the nipple tapered portion, the metal sealing member being disposed in the adapter body to provide a seal between the adapter and the pipe nipple, wherein the adapter body comprises a protruding portion formed to have a constant width between the nipple-coupling threaded portion and the tapered seating portion, wherein the pipe nipple comprises a nipple cylindrical portion having a cylindrical shape and formed to have a width smaller than a width of the protruding portion between the nipple threaded portion and the nipple tapered portion, and wherein the metal sealing member comprises a plurality of sealing member protrusions protruding from an edge of the sealing member body so as to be located on the protruding portion and to contact an outer surface of the nipple cylindrical portion 153).