Provided is a seed treatment agent having excellent control effect on plant diseases and a method for protecting a plant from plant diseases.A seed treatment agent comprising, as an active ingredient, an α-methoxyphenylacetic acid compound represented by formula (1).本發明係提供對植物病害具有優異防治功效的種子處理劑以及保護植物免於植物病害的方法。一種種子處理劑,其包括式(1)所示之α-甲氧基苯基乙酸化合物作為活性成分。