There is provided an ink for printing a conductor pattern on a substrate, including platinum particles, wherein 70% or more of the platinum particles have a particle size of 0.05 to 0.5 μm. Even when the viscosity of the printing ink is controlled to a relatively low level for use in ink-jet printing process, it is possible by such particle size distribution control to prevent sedimentation of the platinum particles and excessive shrinkage of the conductor pattern due to sintering of the platinum particles during firing so that the conductor pattern can attain improved conduction characteristics.