The authors theoretically analyzed the output characteristics of a proposed Hanle-effect spin transistor based on a spin-MOSFET. The device can easily create oscillating Hanle-effect signals by applying an accelerating bias voltage. The behavior of the magnetic field interval of the oscillatory Hanle-effect signals for a sufficiently high accelerating bias is well correlated with the universality of the effective electron mobility in the Si MOS inversion channel, which is useful for revealing spin transport dynamics in the MOS inversion channel.