To increase rigidity by improving Young's modulus by the reduction of CE value and also to prevent the occurrence of shrinkage cavity.

The carbon equivalent (CE value), computed by CE=(Si+P)/3+C according to the contents of C, Si, and P expressing in a mass ratio, is regulated to a value ranging from 3.4 to 4.0%, and the contents of C and Si expressed in the mass ratio satisfy C≥Si/3+2.53, and further, the content of Si expressed in the mass ratio is regulated to ≥1.0%. The occurrence of shrinkage cavity can be effectively prevented while increasing rigidity by raising Young's modulus to ≥180 GPa. The occurrence of shrinkage cavity can be prevented more effectively in the case where Si content is ≤1.4% in particular.