Office workers, particularly those in their 30s, are increasingly stressed and struck by mental health problems, partly because of grueling corporate competition, a new study shows.

More than 61 percent of 218 leading companies surveyed by a think tank said employees with mental health problems have steadily increased over the past few years.

A report by the Mental Health Research Institute of the Japan Productivity Center for Socio-Economic Development said nearly 75 percent of the survey's respondents reported having employees who take leaves lasting longer than one month, up from 67 percent in 2004.

Sixty-one percent of the polled companies said employees in their 30s constituted the largest group with mental health problems, a sharp increase from 41.8 percent in 2002 and 49.3 percent in 2004, according to the biennial survey released last month.

Analysts cited by the study said harsher competition among colleagues due to performance-based pay systems, as well as decreasing communication and mutual support, were the most likely causes.

Many companies underwent a major overhaul to boost efficiency and profitability in the 1990s, when corporate Japan struggled to survive a decadelong economic slump. Firms slashed workforces, stopped routine hiring of college graduates and shifted to merit-based pay systems.

"As the traditional, seniority-based pay system has been largely replaced with performance-based pay and achievement systems . . . colleagues have become rivals," institute analyst Kotaro Kusunoki said.