  We introduce extended chiral transformation, which depends both on
pseudoscalar and diquark fields as parameters and determine its group
structure. Assuming soft symmetry breaking in diquark sector, bosonisation of a
quasi-Goldstone $ud$-diquark is performed. In the chiral limit the $ud$-diquark
mass is defined by the gluon condensate, $m_{ud}\approx 300 MeV$. The diquark
charge radius is $<r^2_{ud}>^{1/2}\approx 0.5 fm$. We consider also the flavour
triplet of scalar diquarks $(ud)$, $(us)$ and $(ds)$ together with pseudoscalar
mesons and calculate diquark masses and decay constants in terms of meson
parameters and the gluon condensate.
