Introduction The objective of this study is to investigate the effect of the distribution of a booklet providing information to fathers during their partners' pregnancies on fathers' and mothers' postpartum mental health and quality of life (QOL), as well as on fathers' childcare participation and living situations.

Methods and analysis This randomised controlled trial will comprise 554 couples consisting of pregnant women due to give birth at an obstetric institution in Aichi Prefecture, Japan and their partners. Participants will be recruited during prenatal check-ups in the third trimester, and those who provide written consent will be allocated randomly to an intervention and a control group. The pregnant women's partners allocated to the intervention group will be given a booklet written for men containing information on childbirth and postnatal period. Its content will include matters such as what preparations the partner should make before birth and tips for housework and childcare as well as how to prevent unintentional injury to the baby. The control group will not receive any intervention. A baseline survey in the third trimester and follow-up surveys at 1 and 3 months post partum will be carried out using self-administered questionnaires. The primary outcome is the proportion of new mothers' partners at risk for paternal depression (Edinburgh Postnatal Depression Scale score ≥8). Secondary outcomes include the risk of postnatal depression in new mothers, QOL of new mothers and their partners, partners' knowledge of and engagement in housework and childcare, marital relations and parenting stress on the part of new mothers.

Ethics and dissemination This study has been approved by the Ethical Committee at the National Center for Child Health and Development, Tokyo, Japan. The results of the study will be widely disseminated as peer-reviewed papers and at international conferences, with the aim of improving public health services in Japan.

Trial registration number UMIN000021475; Pre-results.