Objective Develop an unconstrained sleep respiration method suitable for daily use at home to realize high precision screening of sleep apnea syndrome or other respiratory diseases without affecting normal sleep. Methods A new unconstrained measurement method using sheet-type flexible pressure sensor was proposed. This method obtains the information of respiratory motions of chest and abdomen by measuring the pressure fluctuations of the chest and abdomen acting on the mattress. The experiments were conducted on ten healthy subjects to confirm the effectiveness of the proposed method by comparing the results of the unconstrained measurement and those of respiratory inductance plethysmography (RIP) using band sensors. Results Sheet-type flexible pressure sensor could measure the pressure fluctuations of the chest and abdomen acting on the mattress during respiration and obtain respiratory waveform and respiratory rate. The respiratory rate measured with the sheet-type flexible pressure sensor agreed with those obtained by RIP. However, it was interesting to find that the gender and the lying position greatly affect whether the phases of the motions of the chest and abdomen measured with the flexible sensor differed from those obtained by RIP. For male subjects in supine or lateral lying positions, the pressure fluctuation originating from the motion of the abdomen was in-phase with the output of the abdomen band sensor, while that from the chest was out-of-phase with the output of the chest band sensor. For female subjects in a supine lying position, the pressure caused by the motion of the chest in-phase fluctuated with that of the abdomen; both of these outputs were in-phase with the output of the band sensors. In the case of female subjects in a lateral lying position, the pressure fluctuation from the chest and that from the abdomen were out-of-phase, and the pressure fluctuations of the chest and abdomen were in- and out-of-phase compared to those of the outputs of the band sensors, respectively. Conclusions Sheet-type flexible pressure sensor is effective to monitor unconstraint sleep respiration; this indicates the potential to identify the types of sleep apnea syndrome. But further researches are required to identify the phase difference between the motion of chest and the abdomen, which are coupled with each other.