There is disclosed a head substrate of a printing head detachably mounted on a printer main body, comprising plural external connection terminals individually receiving, from the exterior, a binary logic signal corresponding to whether or not to execute a recording operation, a recording image signal and a clock signal, recording execution means for executing the recording operation according to the recording image signal and the clock signal entered through the external connection terminals, in case the binary logic signal is in a first state, data memory means for and data readout, and memory access means for recognizing the binary logic signal in a second state as an access permission signal and executing the memory access to the data memory means at a timing corresponding to the clock signal when the logic signal is a second state.