Spark plug

A spark plug comprising:

• a cylindrical insulator having an axial hole extending in an axial direction;
• a center electrode held in said axial hole on a leading end side;
• a connecting terminal held in said axial hole on a trailing end side;
• and
• a sealing layer provided in said axial hole and comprising a glass sealing material containing a glass component and a metallic component, said sealing layer including a first sealing layer containing a first glass sealing material and a second sealing layer containing a second glass sealing material laminated in an axial direction of said axial hole, said first sealing layer contacting said center electrode, and said second sealing layer contacting said connecting terminal,
• wherein said second glass sealing material has higher fluidity than that of said first glass sealing material at a temperature higher than a softening point of a glass component contained in said sealing layer,
• wherein the glass component of said sealing layer contains Si, B and an alkali metal comprising at least one of K and Na,
• a content of one of Si and B in said sealing layer is larger than that of any other glass component in said sealing layer, and a content of the other of Si and B in said sealing layer is not larger than the content of said one of Si and B and is larger than that of any other glass component in said sealing layer, and
• said sealing layer satisfies either of the following relationships:
• WB1<WB2, and WA1≦WA2;
• or
• WB1≦WB2, and WA1<WA2,
• wherein WB1 represents a content of B in said first sealing layer in terms of B2O3, WB2 represents a content of B in said second sealing layer in terms of B2O3, WA1 represents a content of A in said first sealing layer in terms of A2O, and WA2 represents a content of A in said second sealing layer in terms of A2O where A represents the alkali metal.