The present invention provides an industrially advantageous process for producing a steroid C.sub.17,20 lyase inhibitor represented by the general formula (I): ##STR00001## and a Reformatsky reagent in a stable form suitable for the process.In the present invention, a compound represented by the general formula (I) is produced by reducing a specific .beta.-hydroxy ester compound derivative or a salt thereof obtained from a specific carbonyl compound in a Reformatsky reaction in the presence of a metal hydride complex and a metal halide, and then subjecting it to a ring-closing reaction. In the above Reformatsky reaction, it is useful to use a stable solution of a compound represented by the general formula BrZnCH.sub.2COOC.sub.2H.sub.5 or a crystal of the compound which is represented by the formula (BrZnCH.sub.2COOC.sub.2H.sub.5.THF).sub.2.