Ignition system for internal combustion engine

An ignition system for an internal combustion engine, comprising:

• a tubular insulating member which has a through-hole;
• a tubular terminal which is mounted in an upper portion of the through-hole of the insulating member;
• an ignition coil which has a first terminal;
• the first terminal being fitted into an upper portion of the terminal so as to be electrically connected to the terminal;
• a coiled spring which is attached to a lower portion of the terminal and is loosely fitted into a lower portion of the through-hole of the insulating member;
• a spark plug which has a second terminal fitted into the through-hole from a lower portion of the insulating member;
• the second terminal being brought into contact with the coiled spring so as to be electrically connected to the coiled spring;
• and
• a buckling preventive portion for reducing an inside diameter of the through-hole of the insulating member, which is provided at a portion of the through-hole between the lower portion of the terminal and an upper portion of the second terminal so as to prevent buckling of an intermediate portion of the coiled spring.