We report on the real-time electrical detection of molecular charges at the ovum surface of sea urchin using the biotransistor. The surface potential at the gate surface of biotransistor decreased after the introduction of sea urchin ova. The negative shift of surface potential indicates the increase of negative charges based on sialic acids, sulphated saccharides and so on at the sea urchin ovum surface contacted with the gate surface of biotransistor. Since sulfated oligosialic acid units with negative charges at the ovum surface of sea urchin behave as receptor for sperm, it might be simple and easy to estimate the fertilization ability of ova for not only sea urchin but also mammals based on the electrical signal of intrinsic molecular charges at the ovum surface by use of the biotransistor. The platform based on the biotransistor is expected to be a direct, simple, label-free and non-invasive detection system in clinical diagnosis for in vitro fertilization (IVF).