Context : Magnetic fields can affect significantly the star formation process .
The theory of the magnetically-driven collapse in a uniform field predicts that initially the contraction happens along the field lines .
When the gravitational pull grows strong enough , the magnetic field lines pinch inwards , giving rise to a characteristic hourglass shape .

Aims : We investigate the magnetic field structure of a young Class 0 object , IRAS 15398-3359 , embedded in the Lupus I cloud .
Previous observations at large scales suggest that this source evolved in an highly magnetised environment .
This object thus appears an ideal candidate to study the magnetically driven core collapse in the low-mass regime .

Methods : We have performed polarisation observations of IRAS 15398-3359 at 214 \mu m using the SOFIA/HAWC+ instrument , thus tracing the linearly polarised thermal emission of cold dust .

Results : Our data unveil a significant bend of the magnetic field lines due to the gravitational pull .
The magnetic field appears ordered and aligned with the large-scale B-field of the cloud and with the outflow direction .
We estimate a magnetic field strength of B = { 78 } \mu G , expected to be accurate within a factor of two .
The measured mass-to-flux parameter is \lambda = { 0.95 } , indicating that the core is in a transcritical regime .

Conclusions :