A signal level clamping apparatus for a CTDM video signal used for a time-base correction apparatus has a clamping circuit supplied with a reproduced color difference signal wherein, between adjacent two horizontal sync. signals, a first and second time-compressed color difference line signals are sequentially arranged with a predetermined interval therebetween, a time-base correction unit supplied with an output from the clamping circuit, a sync. signal separator circuit for separating a reproduced horizontal sync. signal from the reproduced color difference signal supplied thereto, a write clock signal generating circuit for generating a write clock signal synchronized with the reproduced horizontal sync. signal from the sync. signal separator circuit and for applying the write clock signal to the time-base correction unit, and a clamping pulse generating circuit for generating a clamping pulse synchronized with the reproduced horizontal sync. signal from the sync. signal separator circuit and for applying the clamping pulse to the clamping circuit, wherein, in response to the clamping pulse, the clamping circuit clamps a portion of a predetermined level within a space between the first and second time-compressed color difference line signals arranged between the adjacent horizontal sync. signals of the reproduced color difference signal is clamped.