Control valve body of automatic transmission and manufacturing method therefor

By pouring, by means of pressure, melted liquid of magnesium or magnesium alloy into a hollow portion which is formed using a casting mold, a valve body where a channel groove for hydraulic oil for use in a speed change operation of the automatic transmission is formed. In the above, the channel grove for the hydraulic oil is formed due to a convex portion which is formed on the mold so as to project into the hollow portion. As the magnesium has small heat capacity, variation of the temperature of the mold during the cooling and heating cycle can be suppressed. Moreover, as the magnesium melted liquid has no reactivity with respect to the casting mold, the convex portion is not deteriorated. This allows employment of a mold having a thinner convex portion formed thereon, and therefore formation of a valve body having a narrower channel groove.