The explosion mechanism of supernovae (SNe) is a longstanding mystery. Clarifying the multi-dimensional shape of the explosion by observations will give important constraints on the explosion mechanism. Among various kinds observations, polarization measurement is the most direct method to study the geometry of a SN, an extragalactic point source. We have been conducting spectropolarimetric observations of SNe with the Subaru telescope. We have detected a large degree of polarization at the lines of the elements synthesized during the SN explosion. This undoubtedly indicates that the explosion is not spherically symmetric. In addition, we have detected the rotation of the polarization angle across the line. This suggests that there is not a well-defined symmetric axis in the element distribution, and thus, the SN explosion actually has a complex three-dimensional shape. A mechanism driving a three-dimensional explosion, from a presumably spherical or axisymmetric collapse of the star, is necessary for the successful explosion.