A spark plug comprising: a metal shell having a tubular shape; an insulator having an axial hole that extends along an axial line (O), at least a portion of a periphery of the insulator being held by the metal shell ; a center electrode disposed in the axial hole ; and a ground electrode that is fixed to the metal shell and that forms a discharge gap (G) between the ground electrode and the center electrode , wherein at least one of the center electrode and the ground electrode includes a projection having a columnar shape, the projection projecting toward the other of the center electrode and the ground electrode and forming the discharge gap (G), wherein the projection includes a noble metal tip having a cylindrical shape disposed at a side adjacent to the discharge gap (G), and a melted portion disposed at a side of the noble metal tip opposite to the side adjacent to the discharge gap (G), the melted portion being formed by melting the noble metal tip and an electrode base material , wherein a side surface of the projection has a recess formed in the melted portion , characterized in that : the recess is formed only in a portion of the entire periphery of the side surface of the projection , and wherein, when D is a depth of the recess from a side surface of the noble metal tip and R is a diameter of the noble metal tip , the following first condition is satisfied: 0.05 mm ≤ D ≤ 0.3 × R a metal shell having a tubular shape; an insulator having an axial hole that extends along an axial line (O), at least a portion of a periphery of the insulator being held by the metal shell ; a center electrode disposed in the axial hole ; and a ground electrode that is fixed to the metal shell and that forms a discharge gap (G) between the ground electrode and the center electrode , wherein at least one of the center electrode and the ground electrode includes a projection having a columnar shape, the projection projecting toward the other of the center electrode and the ground electrode and forming the discharge gap (G), wherein the projection includes a noble metal tip having a cylindrical shape disposed at a side adjacent to the discharge gap (G), and a melted portion disposed at a side of the noble metal tip opposite to the side adjacent to the discharge gap (G), the melted portion being formed by melting the noble metal tip and an electrode base material , wherein a side surface of the projection has a recess formed in the melted portion , characterized in that : the recess is formed only in a portion of the entire periphery of the side surface of the projection , and wherein, when D is a depth of the recess from a side surface of the noble metal tip and R is a diameter of the noble metal tip , the following first condition is satisfied: 0.05 mm ≤ D ≤ 0.3 × R a noble metal tip having a cylindrical shape disposed at a side adjacent to the discharge gap (G), and a melted portion disposed at a side of the noble metal tip opposite to the side adjacent to the discharge gap (G), the melted portion being formed by melting the noble metal tip and an electrode base material , wherein a side surface of the projection has a recess formed in the melted portion , characterized in that : the recess is formed only in a portion of the entire periphery of the side surface of the projection , and wherein, when D is a depth of the recess from a side surface of the noble metal tip and R is a diameter of the noble metal tip , the following first condition is satisfied: 0.05 mm ≤ D ≤ 0.3 × R the recess is formed only in a portion of the entire periphery of the side surface of the projection , and wherein, when D is a depth of the recess from a side surface of the noble metal tip and R is a diameter of the noble metal tip , the following first condition is satisfied: 0.05 mm ≤ D ≤ 0.3 × R <EOS>