Abstract: A spike tire for a vehicle and which includes a pin receptacle hole formed on the tread surface of the tire; a spike pin retractably protrudable from the pin receptacle hole; a biasing mechanism for continuously urging the spike pin to protrude from the pin receptacle hole; and an actuating member of a shape memory alloy capable of restoring a memorized shape when warmed to a predetermined temperature by the heat generated in the tire as a result of operation of the vehicle on a non-frozen or snow-free road surface, to thereby retract the spike pin into said tire against the action of the biasing mechanism. The spike pin is automatically protruded and retracted depending upon road conditions based on a temperature difference between a frozen or snowed road surface and a non-frozen snow-free road surface.