The width of the inverted Lamb dip in H2CO at 3.51 &mgr;m [51,5(ground state) →60,6(v5=1)] was measured by using a low‐noise He‐Xe laser in an axial magnetic field as the light source. To measure the dip width it was necessary to determine first the range of the magnetic field strength and the maximum frequency deviation of the laser frequency modulation. Based on these results, the power‐broadening characteristics were measured. Finally, the relation between the dip width free from the power broadening and the pressure of H2CO was obtained. The dip width (HWHM) was &Dgr;&ngr;h= (123±24)+(155±31)PF(kHz), wherePFrepresents the H2CO pressure expressed in Pascal.