Young people’s transition to adulthood, not only school to work, but also leaving home and family formation has been prolonging and delaying for the last few decades. Though deterioration of the youth labour market affects their leaving home and family formation seriously, different social security system may affect them differently. Some system with generous. support for young people would absorb the risk in the labour market for leaving home and family formation, but others with poor support would not. We examine the effects of social security for young people’s leaving home and family formation between Japan and UK. Japan has only limited social security for youth, but UK still has moderate ones, though they has been set backed in the last few decades. Our result indicates that, in UK, their social security mitigates young people’s family formation when they are at risk in the labour market, but in Japan, there are scarcely or no effects. Generous social security system is necessity for protects young people’s citizenship.