A bolt for projection welding to a steel sheet part, the bolt comprising: a shaft part having a male screw; a circular enlarged diameter part formed integrally with the shaft part, the enlarged diameter part having a diameter larger than that of the shaft part; a circular fusion bonding projection disposed at a center of the enlarged diameter part on a side opposite to the shaft part, the fusion bonding projection including: (i) an initial fusion part having at an end surface thereof a tapered portion with an inclination angle (θ2), the tapered portion being configured such that the initial fusion part is reduced in height toward an outer periphery thereof; and (ii) a main fusion part continuous with the initial fusion part, a plastic deformation portion disposed between the fusion bonding projection and the enlarged diameter part and having an inclined surface with an inclination angle (θ1), the inclined surface being such that the inclined surface connects a base portion of the fusion bonding projection and an outer peripheral portion of the enlarged diameter part, wherein the plastic deformation portion includes a central circular fusion part disposed at an iner diameter side with respect to the base portion, and an annular softening portion disposed at an outer diameter side with respect to the base portion, the softening portion is gradually reduced in thickness from the base portion toward the outer peripheral portion of the enlarged diameter part, characterized in that the volume of the softening portion is larger than a volume of the fusion part, wherein a ratio of the volume of the softening portion with respect to the volume of the fusion part ranges from 1.2 to 1.8, and the volume of the fusion bonding projection is smaller than the volume of the plastic deformation portion. a shaft part having a male screw; a circular enlarged diameter part formed integrally with the shaft part, the enlarged diameter part having a diameter larger than that of the shaft part; a circular fusion bonding projection disposed at a center of the enlarged diameter part on a side opposite to the shaft part, the fusion bonding projection including: (i) an initial fusion part having at an end surface thereof a tapered portion with an inclination angle (θ2), the tapered portion being configured such that the initial fusion part is reduced in height toward an outer periphery thereof; and (ii) a main fusion part continuous with the initial fusion part, (i) an initial fusion part having at an end surface thereof a tapered portion with an inclination angle (θ2), the tapered portion being configured such that the initial fusion part is reduced in height toward an outer periphery thereof; and (ii) a main fusion part continuous with the initial fusion part, a plastic deformation portion disposed between the fusion bonding projection and the enlarged diameter part and having an inclined surface with an inclination angle (θ1), the inclined surface being such that the inclined surface connects a base portion of the fusion bonding projection and an outer peripheral portion of the enlarged diameter part, wherein the plastic deformation portion includes a central circular fusion part disposed at an iner diameter side with respect to the base portion, and an annular softening portion disposed at an outer diameter side with respect to the base portion, the softening portion is gradually reduced in thickness from the base portion toward the outer peripheral portion of the enlarged diameter part, characterized in that the volume of the softening portion is larger than a volume of the fusion part, wherein a ratio of the volume of the softening portion with respect to the volume of the fusion part ranges from 1.2 to 1.8, and the volume of the fusion bonding projection is smaller than the volume of the plastic deformation portion.