  In a microlensing event, a large magnification occurs at caustic crossing and
provides an opportunity to obtain a stronger signal associated with the object.
In this paper we study the possibility of magnetic field detection in a
microlensing event through the Zeeman effect. We follow the prescription
introduced by \citep{Robinson:1980} which analyses the spectrum of a star in a
Fourier space to deconvolve other broadening mechanism from the actual Zeeman
effect. First we study magnification contrast between source and spot in terms
of spot size and then consider two distinct strategies using modern
spectrographs and perform a Monte Carlo simulation to find the detection
efficiency in magnification-magnetic field plan. The spectral resolution and
signal to noise ratio in each strategies, specify suitable places in this plane
to detect a magnetic field.
  Apart from complexity of magnetic field on a star, we consider a simple model
and propose a fantastic method to detect magnetic field using spectrum of
source stars at caustic crossing.
